mm/slab: move and refactor __kmem_cache_alias()

mm/slab: move and refactor __kmem_cache_alias()

Move __kmem_cache_alias() to slab_common.c since it's called by
__kmem_cache_create_args() and calls find_mergeable() that both
are in this file. We can remove two slab.h declarations and make
them static. Instead declare sysfs_slab_alias() from slub.c so
that __kmem_cache_alias() can keep calling it.

Add args parameter to __kmem_cache_alias() and find_mergeable() instead
of align and ctor. With that we can also move the checks for usersize
and sheaf_capacity there from __kmem_cache_create_args() and make the
result more symmetric with slab_unmergeable().

No functional changes intended.

Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>