netlink: specs: fix duplicate if/then keys in netlink-raw schema

Currently netlink-raw.yaml contains two if keys and two then keys in a
single mapping that enforces a "len" for "pad" members and a "len" or
"struct" for binary members.

During validation PyYAML resolves duplicate keys last-wins, so only the
binary rule survives. Pad has not been validated since commit
bf08f32c8c ("tools/net/ynl: Add support for nested structs") added
the second if/then pair in January 2024.

None of the current specs violate this rule, but this validation should
not be parser dependent and unspecified. Strict YAML validators such as
Red Hat's VS Code YAML extension and Adrien Verge's yamllint will
reject the netlink-raw.yaml schema:

Command:
  $ yamllint Documentation/netlink/netlink-raw.yaml

Output:
  185:13    error    duplication of key "if" in mapping  (key-duplicates)
  189:13    error    duplication of key "then" in mapping  (key-duplicates)

The following invalid netlink family spec will pass validation in the
current ynl tooling:

  # SPDX-License-Identifier: ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause)
  ---
  name: minimal-raw
  doc: Minimal netlink-raw family for schema validation testing.
  protocol: netlink-raw
  protonum: 0

  definitions:
    -
      name: test-struct
      type: struct
      members:
        -
          name: reserved
          type: pad
          # len intentionally omitted

  attribute-sets: []

  operations:
    list: []

Signed-off-by: Taylor Bates <tmbates12@gmail.com>
Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20260914-ynl-robustness-v2-1-c6125003b21e@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/netlink/netlink-raw.yaml

@@ -176,20 +176,23 @@ properties:
               struct:
                 description: Name of the nested struct type.
                 type: string
-            if:
-              properties:
-                type:
-                  const: pad
-            then:
-              required: [ len ]
-            if:
-              properties:
-                type:
-                  const: binary
-            then:
-              oneOf:
-                - required: [ len ]
-                - required: [ struct ]
+            allOf:
+              -
+                if:
+                  properties:
+                    type:
+                      const: pad
+                then:
+                  required: [ len ]
+              -
+                if:
+                  properties:
+                    type:
+                      const: binary
+                then:
+                  oneOf:
+                    - required: [ len ]
+                    - required: [ struct ]
         # End genetlink-legacy
 
   attribute-sets: