selftests: fib_nexthops: Test large scale nexthop flushing

Test that all the nexthops are flushed when a multi-part nexthop dump is
required for the flushing.

Without previous patch:

 # ./fib_nexthops.sh
 TEST: Large scale nexthop flushing                                  [FAIL]

With previous patch:

 # ./fib_nexthops.sh
 TEST: Large scale nexthop flushing                                  [ OK ]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>