slab: Rename slab->__page_flags to slab->flags

Slab has its own reasons for using flag bits; they aren't just
the page bits.  Maybe this won't be the ultimate solution, but
we should be clear that these bits are in use.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20250611155916.2579160-3-willy@infradead.org


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