tools/cgroup/slabinfo: update to work with struct slab

tools/cgroup/slabinfo: update to work with struct slab

After the introduction of the dedicated struct slab to describe slab
pages by commit d122019bf061 ("mm: Split slab into its own type") and
the following removal of the corresponding struct page's fields by
commit 07f910f9b729 ("mm: Remove slab from struct page") the
memcg_slabinfo tool broke. An attempt to run it produces a trace like
this:
Traceback (most recent call last):
  File "/usr/bin/drgn", line 33, in <module>
    sys.exit(load_entry_point('drgn==0.0.16', 'console_scripts', 'drgn')())
  File "/usr/lib64/python3.9/site-packages/drgn/internal/cli.py", line 133, in main
    runpy.run_path(args.script[0], init_globals=init_globals, run_name="__main__")
  File "/usr/lib64/python3.9/runpy.py", line 268, in run_path
    return _run_module_code(code, init_globals, run_name,
  File "/usr/lib64/python3.9/runpy.py", line 97, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "/usr/lib64/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "memcg_slabinfo.py", line 226, in <module>
    main()
  File "memcg_slabinfo.py", line 199, in main
    cache = page.slab_cache
AttributeError: 'struct page' has no member 'slab_cache'

The problem can be fixed by explicitly casting struct page * to struct
slab * for slab pages. The tools works as expected with this fix, e.g.:

cred_jar             776    776    192   21    1 : tunables    0    0    0 : slabdata    547    547      0
kmalloc-cg-32          6      6     32  128    1 : tunables    0    0    0 : slabdata      9      9      0
files_cache            3      3    832   39    8 : tunables    0    0    0 : slabdata      8      8      0
kmalloc-cg-512         1      1    512   32    4 : tunables    0    0    0 : slabdata     10     10      0
task_struct           10     10   6720    4    8 : tunables    0    0    0 : slabdata     63     63      0
mm_struct              3      3   1664   19    8 : tunables    0    0    0 : slabdata      9      9      0
kmalloc-cg-16          1      1     16  256    1 : tunables    0    0    0 : slabdata      8      8      0
pde_opener             1      1     40  102    1 : tunables    0    0    0 : slabdata      8      8      0
anon_vma_chain       375    375     64   64    1 : tunables    0    0    0 : slabdata     81     81      0
radix_tree_node        3      3    584   28    4 : tunables    0    0    0 : slabdata    419    419      0
dentry                98     98    312   26    2 : tunables    0    0    0 : slabdata   1420   1420      0
btrfs_inode            3      3   2368   13    8 : tunables    0    0    0 : slabdata    730    730      0
signal_cache           3      3   1600   20    8 : tunables    0    0    0 : slabdata     17     17      0
sighand_cache          3      3   2240   14    8 : tunables    0    0    0 : slabdata     20     20      0
filp                  90     90    512   32    4 : tunables    0    0    0 : slabdata     95     95      0
anon_vma             214    214    200   20    1 : tunables    0    0    0 : slabdata    162    162      0
kmalloc-cg-1k          1      1   1024   32    8 : tunables    0    0    0 : slabdata     22     22      0
pid                   10     10    256   32    2 : tunables    0    0    0 : slabdata     14     14      0
kmalloc-cg-64          2      2     64   64    1 : tunables    0    0    0 : slabdata      8      8      0
kmalloc-cg-96          3      3     96   42    1 : tunables    0    0    0 : slabdata      8      8      0
sock_inode_cache       5      5   1408   23    8 : tunables    0    0    0 : slabdata     29     29      0
UNIX                   7      7   1920   17    8 : tunables    0    0    0 : slabdata     21     21      0
inode_cache           36     36   1152   28    8 : tunables    0    0    0 : slabdata    680    680      0
proc_inode_cache      26     26   1224   26    8 : tunables    0    0    0 : slabdata     64     64      0
kmalloc-cg-2k          2      2   2048   16    8 : tunables    0    0    0 : slabdata      9      9      0

v2: change naming and count_partial()/count_free()/for_each_slab()
    signatures to work with slabs, suggested by Matthew Wilcox

Fixes: 07f910f9b729 ("mm: Remove slab from struct page")
Reported-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Roman Gushchin <guro@fb.com>
Tested-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/linux-patches/Yg2cKKnIboNu7j+p@carbon.DHCP.thefacebook.com/