Avoid unnecessary calls to Container.Spec()

This call does a deep copy, which is only needed if you want
to modify the return value. Instead we use ctr.ConfigNoCopy().Spec
which is just a pointer dereference.

[NO NEW TESTS NEEDED] Just minor performance effects

Signed-off-by: Alexander Larsson <alexl@redhat.com>

pkg/specgen/generate/container.go

@@ -474,7 +474,7 @@ func ConfigToSpec(rt *libpod.Runtime, specg *specgen.SpecGenerator, contaierID s
 		}
 	}
 	specg.OverlayVolumes = overlay
-	_, mounts := c.SortUserVolumes(c.Spec())
+	_, mounts := c.SortUserVolumes(c.ConfigNoCopy().Spec)
 	specg.Mounts = mounts
 	specg.HostDeviceList = conf.DeviceHostSrc
 	specg.Networks = conf.Networks

pkg/specgen/generate/storage.go

@@ -288,7 +288,7 @@ func getVolumesFrom(volumesFrom []string, runtime *libpod.Runtime) (map[string]s
 
 		// Now we get the container's spec and loop through its volumes
 		// and append them in if we can find them.
-		spec := ctr.Spec()
+		spec := ctr.ConfigNoCopy().Spec
 		if spec == nil {
 			return nil, nil, fmt.Errorf("retrieving container %s spec for volumes-from", ctr.ID())
 		}