Fix for RDMA loopback over VLAN in ibcore.

Implement a more generic solution for detecting loopback.
The problem was that the default netdevice was resolved
for loopback also when VLAN was used. Use real network
device instead of loopback device for bound device
interface.

How to test:
ucmatose -b 127.0.0.1 -p 20090
ucmatose -s 5.6.5.1 -p 20090

Note that RDMA treats the IPv4 and IPv6 loopback
addresses like any address.

MFC after: 1 week
Sponsored by: Mellanox Technologies