netlink: specs: rt-neigh: make sure getneigh is consistent

The consistency check complains replies to do and dump don't match
because dump has no value. It doesn't have to by the schema... but
fixing this in code gen would be more code than adjusting the spec.
This is rare.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250418021706.1967583-11-kuba@kernel.org


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