vsock/loopback: support MSG_ZEROCOPY for transport

Add 'msgzerocopy_allow()' callback for loopback transport.

Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>