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>

Documentation/netlink/specs/rt-link.yaml

@@ -1447,6 +1447,8 @@ attribute-sets:
         type: indexed-array
         sub-type: binary
         display-hint: ipv6
+        checks:
+          exact-len: 16
       -
         name: coupled-control
         type: u8