slab: Add SL_partial flag

Give slab its own name for this flag.  Keep the PG_workingset alias
information in one place.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20250611155916.2579160-4-willy@infradead.org


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