veth: Implement ethtool's get_channels() callback

Libbpf's xsk part calls get_channels() API to retrieve the queue count
of the underlying driver so that XSKMAP is sized accordingly.

Implement that in veth so multi queue scenarios can work properly.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210329224316.17793-14-maciej.fijalkowski@intel.com