mm: Use struct slab in kmem_obj_info()

All three implementations of slab support kmem_obj_info() which reports
details of an object allocated from the slab allocator.  By using the
slab type instead of the page type, we make it obvious that this can
only be called for slabs.

[ vbabka@suse.cz: also convert the related kmem_valid_obj() to folios ]

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Roman Gushchin <guro@fb.com>