IB/ehca: Prevent RDMA-related connection failures on some eHCA2 hardware

Some HW revisions of eHCA2 may cause an RC connection to break if they
received RDMA Reads over that connection before.  This can be
prevented by assuring that, after the first RDMA Read, the QP receives
a new RDMA Read every few million link packets.

Include code into the driver that inserts an empty (size 0) RDMA Read
into the message stream every now and then if the consumer doesn't
post them frequently enough.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>