IB/rxe: Fix sending out loopback packet on netdev interface.

Both prepare4 and prepare6 sets loopback mask in pkt_info structure
instance of skb.  The xmit_packet and other requester side functions
use a pkt_info struct from the stack without the proper mask.  This
results in sending out the packet to the actual netdev device and
loopback functionality is broken.

Modify prepare() to pass its correctly marked pkt_info struct to
prepare4() and prepare6() instead of them using SKB_TO_PKT(skb) and
getting an incorrectly set mask.

Verified with perftest applications.

Signed-off-by: Parav Pandit <pandit.parav@gmail.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>