hammer2 - Fix kmalloc pool blowout on low-memory machines

hammer2 - Fix kmalloc pool blowout on low-memory machines

* Fix a kmalloc pool blown that can occur on low-memory machines due
  to too many disconnected hammer2_inode structures building up.

* Was previously fixed for things like rm -rf and bulk renames,
  but not for setattr (aka chown/chmod -R ops).

Reported-by: gjs278