slab: Add SL_pfmemalloc flag

Give slab its own name for this flag.  Move the implementation from
slab.h to slub.c since it's only used inside slub.c.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20250611155916.2579160-5-willy@infradead.org


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