mm/slab: Dissolve slab_map_pages() in its caller

The function no longer does what its name and comment suggests, and just
sets two struct page fields, which can be done directly in its sole
caller.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>