bgpd: Fix nexthop reg for IPv4 route exchange using GUA IPv6 peering

bgpd: Fix nexthop reg for IPv4 route exchange using GUA IPv6 peering

In the case of IPv4 route exchange using GUA IPv6 peering, the route install
into the FIB involves mapping the immediate next hop to an IPv4 link-local
address and installing neighbor entries for this next hop address. To
accomplish the latter, IPv6 Router Advertisements are exchanged (the next hop
or peer must also have this enabled) and the RAs are dynamically initiated
based on next hop resolution.

However, in the case of a passive connection where the local system has not
initiated anything, no NHT entry is created for the peer, hence RAs were not
getting triggered. Address this by ensuring that a NHT entry is created even
in this situation. This is done at the time the connection becomes established
because the code has other assumptions that a NHT entry will be present only
for the "configured" peer. The API to create the entry ensures there are
no duplicates.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>