slub: add comments to endif pre-processor macros

SLUB allocator makes heavy use of ifdef/endif pre-processor macros.  The
pairing of these statements is at times hard to follow e.g.  if the pair
are further than a screen apart or if there are nested pairs.  We can
reduce cognitive load by adding a comment to the endif statement of form

       #ifdef CONFIG_FOO
       ...
       #endif /* CONFIG_FOO */

Add comments to endif pre-processor macros if ifdef/endif pair is not
immediately apparent.

Link: http://lkml.kernel.org/r/20190402230545.2929-5-tobin@kernel.org


Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Acked-by: Christoph Lameter <cl@linux.com>
Reviewed-by: Roman Gushchin <guro@fb.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>