geneve: support setting IPv6 flow label

This work adds support for setting the IPv6 flow label for geneve per
device and through collect metadata (ip_tunnel_key) frontends. Also here,
the geneve dst cache does not need any special considerations, for the
cases where caches can be used, the label is static per cache.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 6bebc97..249eef9 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -479,6 +479,7 @@
 	IFLA_GENEVE_UDP_CSUM,
 	IFLA_GENEVE_UDP_ZERO_CSUM6_TX,
 	IFLA_GENEVE_UDP_ZERO_CSUM6_RX,
+	IFLA_GENEVE_LABEL,
 	__IFLA_GENEVE_MAX
 };
 #define IFLA_GENEVE_MAX	(__IFLA_GENEVE_MAX - 1)