md: don't record new badblocks for faulty rdev

Faulty will be checked before issuing IO to the rdev, however, rdev can
be faulty at any time, hence it's possible that rdev_set_badblocks()
will be called for faulty rdev. In this case, mddev->sb_flags will be
set and some other path can be blocked by updating super block.

Since faulty rdev will not be accesed anymore, there is no need to
record new babblocks for faulty rdev and forcing updating super block.

Noted this is not a bugfix, just prevent updating superblock in some
corner cases, and will help to slice a bug related to external
metadata[1], testing also shows that devices are removed faster in the
case IO error.

[1] https://lore.kernel.org/all/[e-mail]/

Signed-off-by: Yu Kuai <[e-mail]>
Tested-by: Mariusz Tkaczyk <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Song Liu <[e-mail]>

drivers/md/md.c

···
 {
 	struct mddev *mddev = rdev->mddev;
 	int rv;
+
+	/*
+	 * Recording new badblocks for faulty rdev will force unnecessary
+	 * super block updating. This is fragile for external management because
+	 * userspace daemon may trying to remove this device and deadlock may
+	 * occur. This will be probably solved in the mdadm, but it is safer to
+	 * avoid it.
+	 */
+	if (test_bit(Faulty, &rdev->flags))
+		return 1;
+
 	if (is_new)
 		s += rdev->new_data_offset;
 	else