Zebra: Static NHT fixes

Zebra: Static NHT fixes

When NHT calls rib_process() to be invoked for a prefix, the RIB has already
been marked as having NH changes. The first call to nexthop_active_update
clears this flag and attempts to re-determine if there are any NH changes for
a prefix. However, when the NH is recurisve, this fails. Furthermore, since
NHT has already determined that this RIB has NH changes, there's no need to
ascertain that again. The original patch used static route as the proxy to
skip this call which was incorrect since rib_process can be invoked for
static routes for reasons other than NHT. So, this patch removes the check
for static route and directly checks if the NH changed flag has been set.

Signed-off-by: Dinesh G Dutt <ddutt@cumulusnetworks.com>
Reviewed-by:   Vivek Venkataraman <vivek@cumulusnetworks.com>