f2fs: detect bug_on in f2fs_wait_discard_bios

Add bug_on to detect potential non-empty discard wait list.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1388,6 +1388,8 @@ bool f2fs_wait_discard_bios(struct f2fs_sb_info *sbi)
 
 	/* just to make sure there is no pending discard commands */
 	__wait_all_discard_cmd(sbi, NULL);
+
+	f2fs_bug_on(sbi, atomic_read(&dcc->discard_cmd_cnt));
 	return dropped;
 }