mm/slab: correct return values in comment for _kmem_cache_create()

__kmem_cache_create() returns 0 on success and non-zero on failure.
The comment is wrong in two instances, so fix the first one and remove
the second one. Also make the comment non-doc, because it doesn't
describe an API function, but SLAB-specific implementation.

Signed-off-by: zhaoxinchao <chrisxinchao@outlook.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>