pbrd: Handle GATEWAY_IFINDEX nht conflicts

pbrd: Handle GATEWAY_IFINDEX nht conflicts

In pbrd we did not care if the nexthop interface nexthop tracking
sent us back did not match the one specified with `nexthop [GATEWAY]
[INTERFACE]`. This happened if the gateway was resolvable via a
different interface and the inteface we specified in the config was
unnumbered (no ipv4 address on it) since the default route gets forced
onlink when it gets into zebra.

This patch adds a check to not install the rule if the interface we got
back was different from the specified.

This patch also reworks the nexthop update path to make it a little more
clear what its doing.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>