bcachefs: Fix -Wformat in bch2_btree_key_cache_to_text()

bcachefs: Fix -Wformat in bch2_btree_key_cache_to_text()

When building bcachefs for 32-bit ARM, there is a compiler warning in
bch2_btree_key_cache_to_text() due to use of an incorrect format
specifier:

  fs/bcachefs/btree_key_cache.c:1060:36: error: format specifies type 'size_t' (aka 'unsigned int') but the argument has type 'long' [-Werror,-Wformat]
   1060 |         prt_printf(out, "nr_freed:\t%zu",       atomic_long_read(&c->nr_freed));
        |                                     ~~~         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        |                                     %ld
  fs/bcachefs/util.h:223:54: note: expanded from macro 'prt_printf'
    223 | #define prt_printf(_out, ...)           bch2_prt_printf(_out, __VA_ARGS__)
        |                                                               ^~~~~~~~~~~
  1 error generated.

On 64-bit architectures, size_t is 'unsigned long', so there is no
warning when using %zu but on 32-bit architectures, size_t is
'unsigned int'. Use '%lu' to match the other format specifiers used in
this function for printing values returned from atomic_long_read().

Fixes: 6d799930ce0f ("bcachefs: btree key cache pcpu freedlist")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>