btrfs: document that we don't need get_vnode_name

Change-Id: Iffea088f7234ab7d458ed0cabc4ded0aa50009c2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1617
Reviewed-by: Chế Vũ Gia Hy <[email protected]>
Reviewed-by: Adrien Destugues <[email protected]>

src/add-ons/kernel/file_systems/btrfs/kernel_interface.cpp

@@ -994,7 +994,8 @@ fs_volume_ops gBtrfsVolumeOps = {
 fs_vnode_ops gBtrfsVnodeOps = {
 	/* vnode operations */
 	&btrfs_lookup,
-	NULL,
+	NULL, // btrfs_get_vnode_name - optional, and we can't do better than the
+		// fallback implementation, so leave as NULL.
 	&btrfs_put_vnode,
 	NULL,	// btrfs_remove_vnode,