btrfs: fix error pointer dereference in btrfs_ioctl_rm_dev_v2()

If memdup_user() fails the error handing will crash when it tries
to kfree() an error pointer.  Just return directly because there is
no cleanup required.

Fixes: 1a15eb7 ("btrfs: use btrfs_get_dev_args_from_path in dev removal ioctls")
Reviewed-by: Josef Bacik <josef@toxicpanda.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/ioctl.c

@@ -3188,10 +3188,8 @@ static long btrfs_ioctl_rm_dev_v2(struct file *file, void __user *arg)
 		return -EPERM;
 
 	vol_args = memdup_user(arg, sizeof(*vol_args));
-	if (IS_ERR(vol_args)) {
-		ret = PTR_ERR(vol_args);
-		goto out;
-	}
+	if (IS_ERR(vol_args))
+		return PTR_ERR(vol_args);
 
 	if (vol_args->flags & ~BTRFS_DEVICE_REMOVE_ARGS_MASK) {
 		ret = -EOPNOTSUPP;