slab: move validate_slab_ptr() from check_slab() to its callers

We will want to do the validation earlier in some callers or remove it
completely, so extract it from check_slab() first. No functional change.

Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>