Merge commit '3ccf9ee7' into android-4.9

Synced to the latest commit for merge:
3ccf9ee7 f2fs: run fstrim asynchronously if runtime discard is on

Change-Id: I1bd6c8ae7ae23094ae3c128993a87812f984d7e2
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>