f2fs: catch up to v4.14-rc1

Cherry-picked from upstream-f2fs-stable-linux-4.9.y

Changes include:

commit 30da3a4d ("f2fs: hurry up to issue discard after io interruption")
commit d1c363b4 ("f2fs: fix to show correct discard_granularity in sysfs")
...

commit e6b120d4 ("f2fs/fscrypt: catch up to v4.12")
commit 4d7931d7 ("KEYS: Differentiate uses of rcu_dereference_key() and user_key_payload()")

Signed-off-by: Hyojun Kim <hyojun@google.com>