ext4: always drain queued discard work in ext4_mb_release()

While reviewing recent ext4 patch[1], Sashiko raised the following
concern[2]:

> If the filesystem is initially mounted with the discard option,
> deleting files will populate sbi->s_discard_list and queue
> s_discard_work. If it is then remounted with nodiscard, the
> EXT4_MOUNT_DISCARD flag is cleared, but the pending s_discard_work is
> neither cancelled nor flushed.

[1] https://lore.kernel.org/r/20260319094545.19291-1-qiang.zhang@linux.dev/
[2] https://sashiko.dev/#/patchset/20260319094545.19291-1-qiang.zhang%40linux.dev

The concern was valid, but it had nothing to do with the patch[1].
One of the problems with Sashiko in its current (early) form is that
it will detect pre-existing issues and report it as a problem with the
patch that it is reviewing.

In practice, it would be hard to hit deliberately (unless you are a
malicious syzkaller fuzzer), since it would involve mounting the file
system with -o discard, and then deleting a large number of files,
remounting the file system with -o nodiscard, and then immediately
unmounting the file system before the queued discard work has a change
to drain on its own.

Fix it because it's a real bug, and to avoid Sashiko from raising this
concern when analyzing future patches to mballoc.c.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Fixes: 55cdd0a ("ext4: get discard out of jbd2 commit kthread contex")
Cc: stable@kernel.org

Author: tytso

fs/ext4/mballoc.c

@@ -3893,13 +3893,11 @@ void ext4_mb_release(struct super_block *sb)
 	struct kmem_cache *cachep = get_groupinfo_cache(sb->s_blocksize_bits);
 	int count;
 
-	if (test_opt(sb, DISCARD)) {
-		/*
-		 * wait the discard work to drain all of ext4_free_data
-		 */
-		flush_work(&sbi->s_discard_work);
-		WARN_ON_ONCE(!list_empty(&sbi->s_discard_list));
-	}
+	/*
+	 * wait the discard work to drain all of ext4_free_data
+	 */
+	flush_work(&sbi->s_discard_work);
+	WARN_ON_ONCE(!list_empty(&sbi->s_discard_list));
 
 	group_info = rcu_access_pointer(sbi->s_group_info);
 	if (group_info) {