IB/cm: Do not automatically disconnect XRC TGT QPs

Because an XRC TGT QP can end up being shared among multiple
processes, don't have the ib_cm automatically send a DREQ when the
userspace process that owns the ib_cm_id exits.  Disconnect can be
initiated by the user directly; otherwise, the owner of the XRC INI QP
controls the connection.

Note that as a result of the process exiting, the ib_cm will stop
tracking the XRC connection on the target side.  For the purposes of
disconnecting, this isn't a big deal.  The ib_cm will respond to the
DREQ appropriately.  For other messages, mainly LAP, the CM will
reject the request, since there's no one available to route the
request to.

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