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

Cherry-picked from:
  origin/upstream-f2fs-stable-linux-4.9.y

cea3ae57 ("f2fs: turn down IO priority of discard from background")
0357a0fa ("f2fs: don't split checkpoint in fstrim")
41059095 ("f2fs: issue discard commands proactively in high fs utilization")
3a38cf15 ("f2fs: add fsync_mode=nobarrier for non-atomic files")
8f6a2e7a ("f2fs: let fstrim issue discard commands in lower priority")

Change-Id: Id7bfb0e31103b42b0eb87928d2d6bc34f18f555c
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>