zebra: Re-evaluate the nexthop tracking if flags changed

zebra: Re-evaluate the nexthop tracking if flags changed

Routing protocols are allowed ( and even encouraged ) to modify
the flags that influence the nexthop tracking.  As such when
we modify the tracking of a nexthop to go from, say, connected force
or not we must re-evaluate the nexthop and send the results
up to the interested parties.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>