net: Add netif_is_vxlan()

Add the ability to determine whether a netdev is a VxLAN netdev by
calling the above mentioned function that checks the netdev's
rtnl_link_ops.

This will allow modules to identify netdev events involving a VxLAN
netdev and act accordingly. For example, drivers capable of VxLAN
offload will need to configure the underlying device when a VxLAN netdev
is being enslaved to an offloaded bridge.

Convert nfp to use the newly introduced helper.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>