inactive_anon_is_low: move to vmscan

inactive_anon_is_low: move to vmscan

The inactive_anon_is_low() is called only vmscan.  Then it can move to
vmscan.c

This patch doesn't have any functional change.

Reviewd-by: KAMEZAWA Hiroyuki <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Signed-off-by: KOSAKI Motohiro <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Daisuke Nishimura <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>