[PATCH 1/2] mm: prevent to add a page to swap if may_writepage is unset

This patch prevents to add a page to swap cache unnecessary when
may_writepage is unset so anoymous lru list isn't full of
Dirty/Swapcache page. So VM can isolate pages from anon lru list,
which ends up setting may_writepage to 1 and could swap out
anon lru pages. When OOM triggers, I confirmed swap space was full.

Reported-by: Luigi Semenzato <semenzato@google.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>