netlink: specs: fou: link the type attribute to the encap-type enum

The spec defines encap-type (unspec / direct / gue), but no attribute
references it, so the definition exists only to emit FOU_ENCAP_* into
the uAPI header - while FOU_ATTR_TYPE is exactly that value space:
fou_create() switches on FOU_ENCAP_DIRECT / FOU_ENCAP_GUE and returns
-EINVAL for anything else. Python YNL could not accept or display the
names and the generated C exposed a raw __u8 setter.

The global policy entry becomes NLA_POLICY_MAX(NLA_U8, 2). For add
that only moves the existing fou_create() rejection earlier; del and
get ignore FOU_ATTR_TYPE altogether, so a bogus type there now fails
validation instead of being dropped on the floor.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://patch.msgid.link/20260904191412.3872344-4-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/netlink/specs/fou.yaml

@@ -44,6 +44,7 @@ attribute-sets:
       -
         name: type
         type: u8
+        enum: encap-type
       -
         name: remcsum-nopartial
         type: flag

net/ipv4/fou_nl.c

@@ -16,7 +16,7 @@ const struct nla_policy fou_nl_policy[FOU_ATTR_IFINDEX + 1] = {
 	[FOU_ATTR_PORT] = { .type = NLA_BE16, },
 	[FOU_ATTR_AF] = { .type = NLA_U8, },
 	[FOU_ATTR_IPPROTO] = NLA_POLICY_MIN(NLA_U8, 1),
-	[FOU_ATTR_TYPE] = { .type = NLA_U8, },
+	[FOU_ATTR_TYPE] = NLA_POLICY_MAX(NLA_U8, 2),
 	[FOU_ATTR_REMCSUM_NOPARTIAL] = { .type = NLA_FLAG, },
 	[FOU_ATTR_LOCAL_V4] = { .type = NLA_BE32, },
 	[FOU_ATTR_LOCAL_V6] = NLA_POLICY_EXACT_LEN(16),