f2fs: catch up to v4.14-rc1

This is cherry-picked from upstrea-f2fs-stable-linux-4.4.y.

Changes include:

commit c7fd9e2b ("f2fs: hurry up to issue discard after io interruption")
commit 603dde39 ("f2fs: fix to show correct discard_granularity in sysfs")
...
commit 565f0225 ("f2fs: factor out discard command info into discard_cmd_control")
commit c4cc29d1 ("f2fs: remove batched discard in f2fs_trim_fs")

Change-Id: Icd8a85ac0c19a8aa25cd2591a12b4e9b85bdf1c5
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>