Merge commit '73450231' into android-4.4

Synced to the latest commit for merge:
73450231 f2fs: run fstrim asynchronously if runtime discard is on

Change-Id: Icec09d14f2768dfaa1a0691eac275f68adbf470b
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>