mm/slab_common: provide "slab_merge" option for !IS_ENABLED(CONFIG_SLAB_MERGE_DEFAULT) builds

This is a minor addition to the allocator setup options to provide a
simple way to on demand enable back cache merging for builds that by
default run with CONFIG_SLAB_MERGE_DEFAULT not set.

Link: https://lkml.kernel.org/r/20210319194506.200159-1-aquini@redhat.com


Signed-off-by: Rafael Aquini <aquini@redhat.com>
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>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>