RDS: IB: Change the proxy qp's path_mtu to IB_MTU_256

RDS: IB: Change the proxy qp's path_mtu to IB_MTU_256

The path_mtu of proxy qp of RDS is currently set to IB_MTU_4096, but it
doesn't have much relevance, since the proxy qp is used only for
registration and invalidation of MRs. For the proxy qp to work in most
environments, this patch changes the path_mtu to IB_MTU_256.

Orabug: 26864694

Suggested-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Avinash Repaka <avinash.repaka@oracle.com>
Reviewed-by: Wei Lin Guay <wei.lin.guay@oracle.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>