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.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>