netlink: add a proto specification for FOU

FOU has a reasonably modern Genetlink family. Add a spec.
Acked-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/netlink/specs/fou.yaml

+name: fou
+
+protocol: genetlink-legacy
+
+doc: |
+  Foo-over-UDP.
+
+c-family-name: fou-genl-name
+c-version-name: fou-genl-version
+max-by-define: true
+kernel-policy: global
+
+definitions:
+  -
+    type: enum
+    name: encap_type
+    name-prefix: fou-encap-
+    enum-name:
+    entries: [ unspec, direct, gue ]
+
+attribute-sets:
+  -
+    name: fou
+    name-prefix: fou-attr-
+    attributes:
+      -
+        name: unspec
+        type: unused
+      -
+        name: port
+        type: u16
+        byte-order: big-endian
+      -
+        name: af
+        type: u8
+      -
+        name: ipproto
+        type: u8
+      -
+        name: type
+        type: u8
+      -
+        name: remcsum_nopartial
+        type: flag
+      -
+        name: local_v4
+        type: u32
+      -
+        name: local_v6
+        type: binary
+        checks:
+          min-len: 16
+      -
+        name: peer_v4
+        type: u32
+      -
+        name: peer_v6
+        type: binary
+        checks:
+          min-len: 16
+      -
+        name: peer_port
+        type: u16
+        byte-order: big-endian
+      -
+        name: ifindex
+        type: s32
+
+operations:
+  list:
+    -
+      name: unspec
+      doc: unused
+
+    -
+      name: add
+      doc: Add port.
+      attribute-set: fou
+
+      dont-validate: [ strict, dump ]
+      flags: [ admin-perm ]
+
+      do:
+        request: &all_attrs
+          attributes:
+            - port
+            - ipproto
+            - type
+            - remcsum_nopartial
+            - local_v4
+            - peer_v4
+            - local_v6
+            - peer_v6
+            - peer_port
+            - ifindex
+
+    -
+      name: del
+      doc: Delete port.
+      attribute-set: fou
+
+      dont-validate: [ strict, dump ]
+      flags: [ admin-perm ]
+
+      do:
+        request:  &select_attrs
+          attributes:
+          - af
+          - ifindex
+          - port
+          - peer_port
+          - local_v4
+          - peer_v4
+          - local_v6
+          - peer_v6
+
+    -
+      name: get
+      doc: Get tunnel info.
+      attribute-set: fou
+      dont-validate: [ strict, dump ]
+
+      do:
+        request: *select_attrs
+        reply: *all_attrs
+
+      dump:
+        reply: *all_attrs