zebra: re-install nhg on interface up

zebra: re-install nhg on interface up

Intermittently zebra and kernel are out of sync
when interface flaps and the add's/dels are in
same processing queue and zebra assumes no change in nexthop.
Hence we need to bring in a reinstall to kernel
of the nexthops and routes to sync their states.

Upon interface flap kernel would have deleted NHGs
associated to a interface (the one flapped),
zebra retains NHGs for 3 mins even though upper
layer protocol removes the nexthops (associated NHG).
As part of interface address add ,
re-add singleton NHGs associated to interface.

Ticket: #3173663
Issue: 3173663

Signed-off-by: Ashwini Reddy <ashred@nvidia.com>
Signed-off-by: Chirag Shah <chirag@nvidia.com>