macvtap: Don't leak unreceived packets when we delete a macvtap device.

To avoid leaking packets in the receive queue.  Add a socket destructor
that will run whenever destroy a macvtap socket.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>