doc/netlink/specs: use pad in structs in rt_link

The rt_link spec was using pad1, pad2 attributes in structs which
appears in the ynl output. Replace this with the 'pad' type which
doesn't pollute the output.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231215093720.18774-8-donald.hunter@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/rt_link.yaml

@@ -66,8 +66,9 @@ definitions:
         name: ifi-family
         type: u8
       -
-        name: padding
-        type: u8
+        name: pad
+        type: pad
+        len: 1
       -
         name: ifi-type
         type: u16
@@ -719,11 +720,9 @@ definitions:
         name: family
         type: u8
       -
-        name: pad1
-        type: u8
-      -
-        name: pad2
-        type: u16
+        name: pad
+        type: pad
+        len: 3
       -
         name: ifindex
         type: u32