mm: oom: show unreclaimable slab info when unreclaimable slabs > user memory

The kernel may panic when an oom happens without killable process
sometimes it is caused by huge unreclaimable slabs used by kernel.

Although kdump could help debug such problem, however, kdump is not
available on all architectures and it might be malfunction sometime.
And, since kernel already panic it is worthy capturing such information
in dmesg to aid touble shooting.

Print out unreclaimable slab info (used size and total size) which
actual memory usage is not zero (num_objs * size != 0) when
unreclaimable slabs amount is greater than total user memory (LRU
pages).

The output looks like:

  Unreclaimable slab info:
  Name                      Used          Total
  rpc_buffers               31KB         31KB
  rpc_tasks                  7KB          7KB
  ebitmap_node            1964KB       1964KB
  avtab_node              5024KB       5024KB
  xfs_buf                 1402KB       1402KB
  xfs_ili                  134KB        134KB
  xfs_efi_item             115KB        115KB
  xfs_efd_item             115KB        115KB
  xfs_buf_item             134KB        134KB
  xfs_log_item_desc        342KB        342KB
  xfs_trans               1412KB       1412KB
  xfs_ifork                212KB        212KB

[yang.s@alibaba-inc.com: v11]
  Link: http://lkml.kernel.org/r/1507656303-103845-4-git-send-email-yang.s@alibaba-inc.com
Link: http://lkml.kernel.org/r/1507152550-46205-4-git-send-email-yang.s@alibaba-inc.com


Signed-off-by: Yang Shi <yang.s@alibaba-inc.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>