md: don't allow "-sync" to be set for device in an active array.

If an array is active, devices can be marked 'faulty', but simply
removing the 'sync' flag is wrong.  That only makes sense
for an array which is not active (and is probably only useful
for testing anyway).
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -2642,10 +2642,12 @@ state_store(struct md_rdev *rdev, const char *buf, size_t len)
 		set_bit(In_sync, &rdev->flags);
 		err = 0;
 	} else if (cmd_match(buf, "-insync") && rdev->raid_disk >= 0) {
-		clear_bit(In_sync, &rdev->flags);
-		rdev->saved_raid_disk = rdev->raid_disk;
-		rdev->raid_disk = -1;
-		err = 0;
+		if (rdev->mddev->pers == NULL) {
+			clear_bit(In_sync, &rdev->flags);
+			rdev->saved_raid_disk = rdev->raid_disk;
+			rdev->raid_disk = -1;
+			err = 0;
+		}
 	} else if (cmd_match(buf, "write_error")) {
 		set_bit(WriteErrorSeen, &rdev->flags);
 		err = 0;