Allow faulty devices to be removed from a readonly array.

Removing faulty devices from an array is a two stage process.
First the device is moved from being a part of the active array
to being similar to a spare device. Then it can be removed
by a request from user space.

The first step is currently not performed for read-only arrays,
so the second step can never succeed.

So allow readonly arrays to remove failed devices (which aren't
blocked).

Signed-off-by: NeilBrown <[e-mail]>

drivers/md/md.c

···
 			}
 		}
 
-	if (mddev->degraded) {
+	if (mddev->degraded && ! mddev->ro) {
 		rdev_for_each(rdev, rtmp, mddev) {
 			if (rdev->raid_disk >= 0 &&
 			    !test_bit(In_sync, &rdev->flags) &&
···
 		flush_signals(current);
 	}
 
+	if (mddev->ro && !test_bit(MD_RECOVERY_NEEDED, &mddev->recovery))
+		return;
 	if ( ! (
 		(mddev->flags && !mddev->external) ||
 		test_bit(MD_RECOVERY_NEEDED, &mddev->recovery) ||
···
 
 	if (mddev_trylock(mddev)) {
 		int spares = 0;
+
+		if (mddev->ro) {
+			/* Only thing we do on a ro array is remove
+			 * failed devices.
+			 */
+			remove_and_add_spares(mddev);
+			clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
+			goto unlock;
+		}
 
 		if (!mddev->external) {
 			int did_change = 0;