IB/ipath: Limit # of packets sent without an ACK received

The sender requests an ACK every 1/2 MB to avoid retransmit timeouts that
were causing MVAPICH mod_bw to fail after a predictable number of sends.

Signed-off-by: Bryan O'Sullivan <bryan.osullivan@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>