mm/slab: factor out slab_args_unmergeable()

mm/slab: factor out slab_args_unmergeable()

slab_mergeable() determines whether a slab cache can be merged, but it
should not be used when the cache is not fully created yet.

Extract the pre-cache-creation mergeability checks into
slab_args_unmergeable(), which evaluates kmem_cache_args, slab flags,
and slab_nomerge to determine if a cache will be mergeable before it is
created.

Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20260127103151.21883-2-harry.yoo@oracle.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>