memcg_slabinfo: Fix use of PG_slab

Check PGTY_slab instead of PG_slab.

Fixes: 4ffca5a9 (mm: support only one page_type per page)
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20250611155916.2579160-11-willy@infradead.org


Signed-off-by: Vlastimil Babka <vbabka@suse.cz>