The IB CM provides a message received acknowledged (MRA) message that
can be sent to indicate that a REQ or REP message has been received, but
will require more time to process than the timeout specified by those
messages.  In many cases, the application may not know how long it will
take to respond to a CM message, but the majority of the time, it will
usually respond before a retry has been sent.  Rather than sending an
MRA in response to all messages just to handle the case where a longer
timeout is needed, it is more efficient to queue the MRA for sending in
case a duplicate message is received.

This avoids sending an MRA when it is not needed, but limits the number
of times that a REQ or REP will be resent.  It also provides for a
simpler implementation than generating the MRA based on a timer event.
(That is, trying to send the MRA after receiving the first REQ or REP if
a response has not been generated, so that it is received at the remote
side before a duplicate REQ or REP has been received)

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>