Validate MADs sent by userspace clients for spec compliance with
C13-18.1.1 (prevent duplicate requests and responses sent on the
same port).  Without this, RMPP transactions get aborted because
of duplicate packets.

This patch is similar to that provided by Jack Morgenstein.
Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Jack Morgenstein <jackm@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>