nexthop: Emit a notification when a single nexthop is replaced

The notification is emitted after all the validation checks were
performed, but before the new configuration (i.e., 'struct nh_info') is
pointed at by the old shell (i.e., 'struct nexthop'). This prevents the
need to perform rollback in case the notification is vetoed.

The next patch will also emit a replace notification for all the nexthop
groups in which the nexthop is used.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>