mm/slab: support slab merge

Slab merge is good feature to reduce fragmentation.  If new creating slab
have similar size and property with exsitent slab, this feature reuse it
rather than creating new one.  As a result, objects are packed into fewer
slabs so that fragmentation is reduced.

Below is result of my testing.

* After boot, sleep 20; cat /proc/meminfo | grep Slab

<Before>
Slab: 25136 kB

<After>
Slab: 24364 kB

We can save 3% memory used by slab.

For supporting this feature in SLAB, we need to implement SLAB specific
kmem_cache_flag() and __kmem_cache_alias(), because SLUB implements some
SLUB specific processing related to debug flag and object size change on
these functions.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>