XRC ("eXtended reliable connected") is an IB transport that provides
better scalability by allowing senders to specify which shared receive
queue (SRQ) should be used to receive a message, which essentially
allows one transport context (QP connection) to serve multiple
destinations (as long as they share an adapter, of course).

A few new concepts are introduced to support this.  This patch adds:

 - A new device capability flag, IB_DEVICE_XRC, which low-level
   drivers set to indicate that a device supports XRC.
 - A new object type, XRC domains (struct ib_xrcd), and new verbs
   ib_alloc_xrcd()/ib_dealloc_xrcd().  XRCDs are used to limit which
   XRC SRQs an incoming message can target.

This patch is derived from work by Jack Morgenstein <jackm@dev.mellanox.co.il>.

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