Use extensions to define XRC support

Use extensions to define XRC support

Define a common libibverbs extension to support XRC.

XRC introduces several new concepts and structures:

XRC domains: xrcd's are a type of protection domain used to
associate shared receive queues with xrc queue pairs.  Since
xrcd are meant to be shared among multiple processes, we
introduce new APIs to open/close xrcd's.

XRC shared receive queues: xrc srq's are similar to normal
srq's, except that they are bound to an xrcd, rather
than to a protection domain.  Based on the current spec
and implementation, they are only usable with xrc qps.  To
support xrc srq's, we extend the existing srq_init_attr
structure to include an srq type and other needed information.
The extended fields are ignored unless extensions are being
used to support existing applications.

XRC queue pairs: xrc defines two new types of QPs.  The
initiator, or send-side, xrc qp behaves similar to a send-
only RC qp.  xrc send qp's are managed through the existing
QP functions.  The send_wr structure is extended in a back-
wards compatible way to support posting sends on a send xrc
qp, which require specifying the remote xrc srq.

The target, or receive-side, xrc qp behaves differently
than other implemented qp's.  A recv xrc qp can be created,
modified, and destroyed like other qp's through the existing
calls.  The qp_init_attr structure is extended for xrc qp's,
with extension support dependent upon the qp_type being
defined correctly.

Because xrc recv qp's are bound to an xrcd, rather than a pd,
it is intended to be used among multiple processes.  Any process
with access to an xrcd may allocate and connect an xrc recv qp.
The actual xrc recv qp is allocated and managed by the kernel.
If the owning process explicit destroys the xrc recv qp, it is
destroyed.  However, if the xrc recv qp is left open when the
user process exits or closes its device, then the lifetime of
the xrc recv qp is bound with the lifetime of the xrcd.

The user to kernel ABI is extended to account for opening/
closing the xrcd and the creation of the extended srq type.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>