FROMLIST: f2fs: run fstrim asynchronously if runtime discard is on

Cherry-picked from:
        origin/upstream-f2fs-stable-linux-3.18.y

We don't need to wait for whole bunch of discard candidates in fstrim, since
runtime discard will issue them in idle time.

Change-Id: I7dab8241370ccbb2222a513fadc50d1b479b32c2
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>