md/raid10: don't BUG_ON() in raise_barrier()

If raise_barrier() is called the first time in raid10_sync_request(), which
means the first non-normal io is handled, raise_barrier() should wait for
all dispatched normal io to be done. This ensures that normal io won't
starve.

However, BUG_ON() if this is broken is too aggressive. This patch replace
BUG_ON() with WARN and fall back to not force.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230310073855.1337560-4-yukuai1@huaweicloud.com

drivers/md/raid10.c

@@ -952,7 +952,9 @@ static void flush_pending_writes(struct r10conf *conf)
 static void raise_barrier(struct r10conf *conf, int force)
 {
 	write_seqlock_irq(&conf->resync_lock);
-	BUG_ON(force && !conf->barrier);
+
+	if (WARN_ON_ONCE(force && !conf->barrier))
+		force = false;
 
 	/* Wait until no block IO is waiting (unless 'force') */
 	wait_event_barrier(conf, force || !conf->nr_waiting);