netlink: specs: rt-link: add an attr layer around alt-ifname

alt-ifname attr is directly placed in requests (as an alternative
to ifname) but in responses its wrapped up in IFLA_PROP_LIST
and only there is may be multi-attr. See rtnl_fill_prop_list().

Fixes: b2f63d904e ("doc/netlink: Add spec for rt link messages")
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250414211851.602096-6-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/rt_link.yaml

@@ -1113,11 +1113,10 @@ attribute-sets:
       -
         name: prop-list
         type: nest
-        nested-attributes: link-attrs
+        nested-attributes: prop-list-link-attrs
       -
         name: alt-ifname
         type: string
-        multi-attr: true
       -
         name: perm-address
         type: binary
@@ -1163,6 +1162,13 @@ attribute-sets:
       -
         name: netns-immutable
         type: u8
+  -
+    name: prop-list-link-attrs
+    subset-of: link-attrs
+    attributes:
+      -
+        name: alt-ifname
+        multi-attr: true
   -
     name: af-spec-attrs
     attributes:
@@ -2453,7 +2459,6 @@ operations:
             - min-mtu
             - max-mtu
             - prop-list
-            - alt-ifname
             - perm-address
             - proto-down-reason
             - parent-dev-name