mm/slab: reverse iteration on find_mergeable()

Unlike SLUB, sometimes, object isn't started at the beginning of the slab
in the SLAB.  This causes the unalignment problem when after slab merging
is supported by commit 12220dea ("mm/slab: support slab merge").
Alignment mismatch check is introduced ("mm/slab: fix unalignment problem
on Malta with EVA due to slab merge") to prevent merge in this case.

This causes undesirable result that merging happens between infrequently
used kmem_caches if there are kmem_caches with same size and is 256 bytes,
are merged into pool_workqueue rather than kmalloc-256, because
kmem_caches for kmalloc are at the tail of the list.

To prevent this situation, this patch reverses iteration order in
find_mergeable() to find frequently used kmem_caches.  This change helps
to merge kmem_cache to frequently used kmem_caches, such as kmalloc
kmem_caches.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: 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>