FROMLIST: f2fs: early updates queued for v4.18-rc1

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

0643d71e ("f2fs: turn down IO priority of discard from background")
6d854622 ("f2fs: don't split checkpoint in fstrim")
da639d16 ("f2fs: issue discard commands proactively in high fs utilization")
413fa22b ("f2fs: add fsync_mode=nobarrier for non-atomic files")
d7a1b256 ("f2fs: let fstrim issue discard commands in lower priority")

Change-Id: Ic83898da3eb921238ec154e839120e825325925c
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>