IB/mthca: Optimize large messages on Sinai HCAs

Sinai (one-port PCI Express) HCAs get improved throughput for messages
bigger than 80 KB in DDR mode if memory keys are formatted in a
specific way.  The enhancement only works if the memory key table is
smaller than 2^24 entries.  For larger tables, the enhancement is off
and a warning is printed (to avoid silent performance loss).

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Michael Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>