netlink: specs: fou: change local-v6/peer-v6 check

While updating the binary min-len implementation, I noticed that
the only user, should AFAICT be using exact-len instead.

In net/ipv4/fou_core.c FOU_ATTR_LOCAL_V6 and FOU_ATTR_PEER_V6
are only used for singular IPv6 addresses, and there are AFAICT
no known implementations trying to send more, it therefore
appears safe to change it to an exact-len policy.

This patch therefore changes the local-v6/peer-v6 attributes to
use an exact-len check, instead of a min-len check.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250902154640.759815-2-ast@fiberby.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/fou.yaml

@@ -52,7 +52,7 @@ attribute-sets:
         name: local-v6
         type: binary
         checks:
-          min-len: 16
+          exact-len: 16
       -
         name: peer-v4
         type: u32
@@ -60,7 +60,7 @@ attribute-sets:
         name: peer-v6
         type: binary
         checks:
-          min-len: 16
+          exact-len: 16
       -
         name: peer-port
         type: u16

net/ipv4/fou_nl.c

@@ -18,9 +18,9 @@ const struct nla_policy fou_nl_policy[FOU_ATTR_IFINDEX + 1] = {
 	[FOU_ATTR_TYPE] = { .type = NLA_U8, },
 	[FOU_ATTR_REMCSUM_NOPARTIAL] = { .type = NLA_FLAG, },
 	[FOU_ATTR_LOCAL_V4] = { .type = NLA_U32, },
-	[FOU_ATTR_LOCAL_V6] = { .len = 16, },
+	[FOU_ATTR_LOCAL_V6] = NLA_POLICY_EXACT_LEN(16),
 	[FOU_ATTR_PEER_V4] = { .type = NLA_U32, },
-	[FOU_ATTR_PEER_V6] = { .len = 16, },
+	[FOU_ATTR_PEER_V6] = NLA_POLICY_EXACT_LEN(16),
 	[FOU_ATTR_PEER_PORT] = { .type = NLA_BE16, },
 	[FOU_ATTR_IFINDEX] = { .type = NLA_S32, },
 };