slab: use memdesc_nid()

We no longer need to convert from slab to folio to get the nid, we can ask
memdesc_nid() for the nid directly.

Link: https://lkml.kernel.org/r/20250805172307.1302730-7-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>