vmscan,memcg: reintroduce sc->may_swap

vmscan,memcg: reintroduce sc->may_swap

Commit a6dc60f8975ad96d162915e07703a4439c80dcf0 ("vmscan: rename
sc.may_swap to may_unmap") removed the may_swap flag, but memcg had used
it as a flag for "we need to use swap?", as the name indicate.

And in the current implementation, memcg cannot reclaim mapped file
caches when mem+swap hits the limit.

re-introduce may_swap flag and handle it at get_scan_ratio().  This
patch doesn't influence any scan_control users other than memcg.

Signed-off-by: KOSAKI Motohiro <[email protected]>
Signed-off-by: Daisuke Nishimura <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>