mm: Use owner_priv bit for PageSwapCache, valid when PageSwapBacked

mm: Use owner_priv bit for PageSwapCache, valid when PageSwapBacked

A page is not added to the swap cache without being swap backed,
so PageSwapBacked mappings can use PG_owner_priv_1 for PageSwapCache.

Signed-off-by: Nicholas Piggin <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Bob Peterson <[email protected]>
Cc: Steven Whitehouse <[email protected]>
Cc: Andrew Lutomirski <[email protected]>
Cc: Andreas Gruenbacher <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>