virtiofs: clean up error handling in virtio_fs_get_tree()

commit 833c5a42 upstream.

Avoid duplicating error cleanup.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Yang Bo <yb203166@antfin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>