Merge remote-tracking branch 'origin/upstream-f2fs-stable-linux-4.9.y' into android-4.9

* origin/upstream-f2fs-stable-linux-4.9.y:
  ext4: fix build warning

Change-Id: Iebb5a46a921ff92305d84330afb8ff2e7946a18b
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>