virtio-net: zero req->mhdr.num_buffers

Zero req->mhdr.num_buffers in Tx path when MRG_RXBUF is negotiated as virtio spec demands.

Reviewed-by Zhi Yong Wu <zwu.kernel@gmail.com>
Reviewed-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>