IB/rxe: Replace av->network_type with skb->protocol

In the function rxe_init_packet, based on av->network_type, skb->protocol
is set to ipv4 or ipv6. The functions rxe_prepare and rxe_send are called
after the functin rxe_init_packet.  So in these functions,
av->network_type can be replaced with skb->protocol.

The functions are in the xmit fast path. So with skb->protocol, the
performance will be better.

Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>