f2fs: stop gc/discard thread after fs shutdown

Once filesystem shuts down, daemons like gc/discard thread should be
aware of it, and do exit, in addtion, drop all cached pending discard
commands and turn off real-time discard mode.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>