RDMA/rxe: Make rxe_loopback match rxe_send behavior

RDMA/rxe: Make rxe_loopback match rxe_send behavior

The rxe send path currently counts the number of skbs outstanding between
the rxe driver and the ethernet driver to prevent too many packets to
accumulate waiting to send. This patch makes the local loopback path
behave the same way. The loopback path forwards the packets to the receive
path which will eventually call kfree_skb on all packets and drop the qp
references. This makes the loopback path more useful for software testing.

Link: https://lore.kernel.org/r/20240329145513.35381-13-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>