This patch resolves kernel panic for xfstests/081, caused by recent f2fs_bug_on

 f2fs: add f2fs_bug_on in __remove_discard_cmd

For fixing, we will stop gc/discard thread in prior in ->kill_sb in order to
avoid referring and releasing race among them.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>