f2fs: add f2fs_bug_on in __remove_discard_cmd

Recently, discard related codes have changed a lot, so add f2fs_bug_on to
detect potential bug.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -739,6 +739,8 @@ static void __remove_discard_cmd(struct f2fs_sb_info *sbi,
 {
 	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
 
+	f2fs_bug_on(sbi, dc->ref);
+
 	if (dc->error == -EOPNOTSUPP)
 		dc->error = 0;