slab: wrap debug slab validation in validate_slab_ptr()

This will make it clear where we currently cast struct slab to folio
only to check the slab type, and allow to change the implementation
later with memdesc conversion.

For now use a struct page based implementation instead of struct folio
to be compatible with further upcoming changes.

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