netlink: specs: rt-link: make bond's ipv6 address attribute fixed size

ns-ip6-target is an indexed-array. Codegen for variable size binary
array would be a bit tedious, tell C that we know the size of these
attributes, since they are IPv6 addrs.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250418021706.1967583-8-kuba@kernel.org


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