VSOCK: Add vsockmon device

Add vsockmon virtual network device that receives packets from the vsock
transports and exposes them to user space.

Based on the nlmon device.

Signed-off-by: Gerard Garcia <ggarcia@deic.uab.cat>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>