nhrp, zebra, lib: pass exact received neighbor state value to nhrp

nhrp, zebra, lib: pass exact received neighbor state value to nhrp

As NHRP expects some notification of neighboring entries on GRE
interface, when a new interface notification is encountered, the
exact neighbor state flag is found. Previously, the flag passed
to the upper layer was forced to NDM_STATE which is REACHABLE,
as can be seen on below trace:

2021/08/25 10:58:39 NHRP: [QQ0NK-1H449] Netlink: new-neigh 102.1.1.1 dev gre1 lladdr 10.125.0.2 nud 0x2 cache used 1 type 5

When passing the real value, NHRP received an other value like STALE.

2021/08/25 11:28:44 NHRP: [QQ0NK-1H449] Netlink: new-neigh 102.1.1.1 dev gre1 lladdr 10.125.0.2 nud 0x4 cache used 0 type 5

This flag is important for NHRP, as it permits to monitor the link
layer of NHRP entries.

Fixes: d603c0774eba ("nhrp, zebra, lib: enforce usage of zapi_neigh_ip structure")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>