nhrpd: fix dont consider incomplete L2 entry

nhrpd: fix dont consider incomplete L2 entry

Sometimes, NHRP receives L2 information on a cache entry with the
0.0.0.0 IP address. NHRP considers it as valid and updates the binding
with the new IP address.

> Feb 09 20:09:54 aws-sin-vpn01 nhrpd[2695]: [QQ0NK-1H449] Netlink: new-neigh 10.2.114.238 dev dmvpn1 lladdr 162.251.180.10 nud 0x2 cache used 0 type 4
> Feb 09 20:10:35 aws-sin-vpn01 nhrpd[2695]: [QQ0NK-1H449] Netlink: new-neigh 10.2.114.238 dev dmvpn1 lladdr 162.251.180.10 nud 0x4 cache used 1 type 4
> Feb 09 20:10:48 aws-sin-vpn01 nhrpd[2695]: [QQ0NK-1H449] Netlink: del-neigh 10.2.114.238 dev dmvpn1 lladdr 162.251.180.10 nud 0x4 cache used 1 type 4
> Feb 09 20:10:49 aws-sin-vpn01 nhrpd[2695]: [QQ0NK-1H449] Netlink: who-has 10.2.114.238 dev dmvpn1 lladdr (unspec) nud 0x1 cache used 1 type 4
> Feb 09 20:10:49 aws-sin-vpn01 nhrpd[2695]: [QVXNM-NVHEQ] Netlink: update binding for 10.2.114.238 dev dmvpn1 from c 162.251.180.10 peer.vc.nbma 162.251.180.10 to lladdr (unspec)
> Feb 09 20:10:49 aws-sin-vpn01 nhrpd[2695]: [QQ0NK-1H449] Netlink: new-neigh 10.2.114.238 dev dmvpn1 lladdr 0.0.0.0 nud 0x2 cache used 1 type 4
> Feb 09 20:11:30 aws-sin-vpn01 nhrpd[2695]: [QQ0NK-1H449] Netlink: new-neigh 10.2.114.238 dev dmvpn1 lladdr 0.0.0.0 nud 0x4 cache used 1 type 4

Actually, the 0.0.0.0 IP addressed mentiones in the 'who-has' message is
wrong because the nud state value means that value is incomplete and
should not be handled as a valid entry. Instead of considering it, fix
this by by invalidating the current binding. This step is necessary in
order to permit NHRP to trigger resolution requests again.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>