kernel - Use kmalloc_obj for more zones - step 1b

kernel - Use kmalloc_obj for more zones - step 1b

* Use kmalloc_obj for M_FILE (struct file).

* Use kmalloc_obj for hammer2_io structures.

* Use kmalloc_obj for nfsnode structures, and make it per-mount.

  I don't know if its a good idea to make it per-mount or not.
  As the number of nodes is limited by kern.maxvnodes, it might
  be better to have a global M_NFSNODE kmalloc_obj zone instead
  of a per-mount one.

  (same goes with hammer2_io and hammer2_inode, which are also
  per-mount atm).