vxlan: vxlan_core: Support FDB flushing by nexthop ID

Add support for flush VXLAN FDB entries by nexthop ID.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>