cifs: revalidate mapping prior to satisfying read_iter request with cache=loose

Before satisfying a read with cache=loose, we should always check
that the pagecache is valid before allowing a read to be satisfied
out of it.

Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Jeff Layton <jlayton@poochiereds.net>
Signed-off-by: Steve French <smfrench@gmail.com>