mm/slub.c: add __printf verification to slab_err()

mm/slub.c: add __printf verification to slab_err()

__printf is useful to verify format and arguments.  Remove the following
warning (with W=1):

  mm/slub.c:721:2: warning: function might be possible candidate for `gnu_printf' format attribute [-Wsuggest-attribute=format]

Link: http://lkml.kernel.org/r/20180505200706.19986-1-malat@debian.org
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>