selftests: fib_nexthops: Test cleanup of FDB entries following nexthop deletion

Commit c7cdbe2e

 ("vxlan: support for nexthop notifiers") registered
a listener in the VXLAN driver to the nexthop notification chain. Its
purpose is to cleanup FDB entries that use a nexthop that is being
deleted.

Test that such FDB entries are removed when the nexthop group that they
use is deleted. Test that entries are not deleted when a single nexthop
in the group is deleted.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>