RDMA/rxe: Correct skb on loopback path

rxe_net.c sends packets at the IP layer with skb->data pointing at the IP
header but receives packets from a UDP tunnel with skb->data pointing at
the UDP header.  On the loopback path this was not correctly accounted
for.  This patch corrects for this by using sbk_pull() to strip the IP
header from the skb on received packets.

Fixes: 8700e3e7 ("Soft RoCE driver")
Link: https://lore.kernel.org/r/20210128182301.16859-1-rpearson@hpe.com


Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>