slub: record page flag overlays explicitly

slub: record page flag overlays explicitly

SLUB reuses two page bits for internal purposes, it overlays PG_active and
PG_error.  This is hidden away in slub.c.  Document these overlays
explicitly in the main page-flags enum along with all the others.

Signed-off-by: Andy Whitcroft <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Matt Mackall <[email protected]>
Cc: Nick Piggin <[email protected]>
Tested-by: KOSAKI Motohiro <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Jeremy Fitzhardinge <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>