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: b2f63d90 ("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>