xfrm: lwtunnel: add lwtunnel support for xfrm interfaces in collect_md mode

Allow specifying the xfrm interface if_id and link as part of a route
metadata using the lwtunnel infrastructure.

This allows for example using a single xfrm interface in collect_md
mode as the target of multiple routes each specifying a different if_id.

With the appropriate changes to iproute2, considering an xfrm device
ipsec1 in collect_md mode one can for example add a route specifying
an if_id like so:

ip route add <SUBNET> dev ipsec1 encap xfrm if_id 1

In which case traffic routed to the device via this route would use
if_id in the xfrm interface policy lookup.

Or in the context of vrf, one can also specify the "link" property:

ip route add <SUBNET> dev ipsec1 encap xfrm if_id 1 link_dev eth15

Note: LWT_XFRM_LINK uses NLA_U32 similar to IFLA_XFRM_LINK even though
internally "link" is signed. This is consistent with other _LINK
attributes in other devices as well as in bpf and should not have an
effect as device indexes can't be negative.

Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/uapi/linux/lwtunnel.h

@@ -15,6 +15,7 @@ enum lwtunnel_encap_types {
 	LWTUNNEL_ENCAP_SEG6_LOCAL,
 	LWTUNNEL_ENCAP_RPL,
 	LWTUNNEL_ENCAP_IOAM6,
+	LWTUNNEL_ENCAP_XFRM,
 	__LWTUNNEL_ENCAP_MAX,
 };
 
@@ -111,4 +112,13 @@ enum {
 
 #define LWT_BPF_MAX_HEADROOM 256
 
+enum {
+	LWT_XFRM_UNSPEC,
+	LWT_XFRM_IF_ID,
+	LWT_XFRM_LINK,
+	__LWT_XFRM_MAX,
+};
+
+#define LWT_XFRM_MAX (__LWT_XFRM_MAX - 1)
+
 #endif /* _UAPI_LWTUNNEL_H_ */