rados: support fetching the cluster from the Conn

The cephfs function ceph_create_from_rados requires the cluster_t
value, which is current private to the Conn type. Alias the cluster_t
type and allow other pkgs to get it if needed.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

rados/conn.go

@@ -30,6 +30,14 @@ type Conn struct {
 	connected bool
 }
 
+// ClusterRef represents a fundamental RADOS cluster connection.
+type ClusterRef C.rados_t
+
+// Cluster returns the underlying RADOS cluster reference for this Conn.
+func (c *Conn) Cluster() ClusterRef {
+	return ClusterRef(c.cluster)
+}
+
 // PingMonitor sends a ping to a monitor and returns the reply.
 func (c *Conn) PingMonitor(id string) (string, error) {
 	c_id := C.CString(id)