neighbor: Use nda_policy for validating attributes in adds and dump requests

Add NDA_PROTOCOL to nda_policy and use the policy for attribute parsing and
validation for adding neighbors and in dump requests. Remove the now duplicate
checks on nla_len.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>