virtio_fs: allow idmapped mounts

Allow idmapped mounts for virtiofs.
It's absolutely safe as for virtiofs we have the same
feature negotiation mechanism as for classical fuse
filesystems. This does not affect any existing
setups anyhow.

virtiofsd support:
https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/245

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: mihalicyn

fs/fuse/virtio_fs.c

@@ -1613,6 +1613,7 @@ static struct file_system_type virtio_fs_type = {
 	.name		= "virtiofs",
 	.init_fs_context = virtio_fs_init_fs_context,
 	.kill_sb	= virtio_kill_sb,
+	.fs_flags	= FS_ALLOW_IDMAP,
 };
 
 static int virtio_fs_uevent(const struct kobject *kobj, struct kobj_uevent_env *env)