Merge commit '787f7fac' into android-4.14

Synced to the latest commit for merge:
787f7fac f2fs: run fstrim asynchronously if runtime discard is on

Change-Id: I1e316c9f82f88dc3c0677373ce8f29a8e81b7cc1
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>