Merge commit 'f264f5e3' into android-3.18

Synced to the latest commit for merge:
f264f5e3 f2fs: run fstrim asynchronously if runtime discard is on

Change-Id: I95a3b0404905f6fe8420ff4336feda3f50a911fa
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>