RDMA/core: Add XRC domain support

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>