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>
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 47b6595..199a77a9 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -1388,6 +1388,8 @@
 
 	/* 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;
 }