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

Cherry-picked from:
  origin/upstream-f2fs-stable-linux-4.14.y

72497b8d ("f2fs: turn down IO priority of discard from background")
cfe6e2f6 ("f2fs: don't split checkpoint in fstrim")
596c9767 ("f2fs: issue discard commands proactively in high fs utilization")
0fbb04e9 ("f2fs: add fsync_mode=nobarrier for non-atomic files")
13e34643 ("f2fs: let fstrim issue discard commands in lower priority")

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