btrfs: drop the lock on error in btrfs_dev_replace_cancel

We should drop the lock on this error path.  This has been found by a
static tool.

The lock needs to be released, it's there to protect access to the
dev_replace members and is not supposed to be left locked. The value of
state that's being switched would need to be artifically changed to an
invalid value so the default: branch is taken.

Fixes: d189dd70 ("btrfs: fix use-after-free due to race between replace start and cancel")
CC: stable@vger.kernel.org # 5.0+
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/dev-replace.c

@@ -862,6 +862,7 @@ int btrfs_dev_replace_cancel(struct btrfs_fs_info *fs_info)
 			btrfs_destroy_dev_replace_tgtdev(tgt_device);
 		break;
 	default:
+		up_write(&dev_replace->rwsem);
 		result = -EINVAL;
 	}