mm: Make SLAB_MERGE_DEFAULT depend on SL[AU]B

SLOB always manage objects of different caches in same page regardless of
SLAB_MERGE_DEFAULT. Because it has no effect on SLOB, make it depend on
SLAB || SLUB.

Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/20211225060921.13584-1-42.hyeyoo@gmail.com