mm, slub: make reclaim_account attribute read-only

The attribute reflects the SLAB_RECLAIM_ACCOUNT cache flag.  It's not
clear why this attribute was writable in the first place, as it's tied to
how the cache is used by its creator, it's not a user tunable.
Furthermore:

- it affects slab merging, but that's not being checked while toggled
- if affects whether __GFP_RECLAIMABLE flag is used to allocate page, but
  the runtime toggle doesn't update allocflags
- it affects cache_vmstat_idx() so runtime toggling might lead to incosistency
  of NR_SLAB_RECLAIMABLE and NR_SLAB_UNRECLAIMABLE

Thus make it read-only.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Roman Gushchin <guro@fb.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Jann Horn <jannh@google.com>
Cc: Vijayanand Jitta <vjitta@codeaurora.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/20200610163135.17364-6-vbabka@suse.cz


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>