virtiofs: fix userns

get_user_ns() is done twice (once in virtio_fs_get_tree() and once in
fuse_conn_init()), resulting in a reference leak.

Also looks better to use fsc->user_ns (which *should* be the
current_user_ns() at this point).
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/virtio_fs.c

@@ -1443,8 +1443,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
 	if (!fm)
 		goto out_err;
 
-	fuse_conn_init(fc, fm, get_user_ns(current_user_ns()),
-		       &virtio_fs_fiq_ops, fs);
+	fuse_conn_init(fc, fm, fsc->user_ns, &virtio_fs_fiq_ops, fs);
 	fc->release = fuse_free_conn;
 	fc->delete_stale = true;
 	fc->auto_submounts = true;