Btrfs: fix unlock order in btrfs_ioctl_rm_dev

Fix unlock order in btrfs_ioctl_rm_dev().

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index fcf1b1b..f5c1c15 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2319,8 +2319,8 @@
 	kfree(vol_args);
 out:
 	mutex_unlock(&root->fs_info->volume_mutex);
-	mnt_drop_write_file(file);
 	atomic_set(&root->fs_info->mutually_exclusive_operation_running, 0);
+	mnt_drop_write_file(file);
 	return ret;
 }