cgroup: reduce complexity of controller disabling

This patch reduces the complexity of the filtering algorithm in
virCgroupDetect by first correcting the controller mask and then
checking for potential co-mounts without any correlating
controller mask modifications.

If you agree that this patch removes complexity and improves
readability it could simply be squashed into the first patch
of this series.
Signed-off-by: NBoris Fiuczynski <fiuczy@linux.vnet.ibm.com>
Reviewed-by: NBjoern Walk <bwalk@linux.vnet.ibm.com>
Reviewed-by: NMarc Hartmayer <mhartmay@linux.vnet.ibm.com>

src/util/vircgroup.c

@@ -656,11 +656,8 @@ virCgroupDetect(virCgroupPtr group,
 
     if (controllers >= 0) {
         VIR_DEBUG("Filtering controllers %d", controllers);
+        /* First mark requested but non-existing controllers to be ignored */
         for (i = 0; i < VIR_CGROUP_CONTROLLER_LAST; i++) {
-            VIR_DEBUG("Controller '%s' wanted=%s, mount='%s'",
-                      virCgroupControllerTypeToString(i),
-                      (1 << i) & controllers ? "yes" : "no",
-                      NULLSTR(group->controllers[i].mountPoint));
             if (((1 << i) & controllers)) {
                 /* Remove non-existent controllers  */
                 if (!group->controllers[i].mountPoint) {
@@ -668,12 +665,15 @@ virCgroupDetect(virCgroupPtr group,
                               virCgroupControllerTypeToString(i));
                     controllers &= ~(1 << i);
                 }
-            } else {
-                if (!group->controllers[i].mountPoint) {
-                    /* without controller co-mounting is impossible */
-                    continue;
-                }
-
+            }
+        }
+        for (i = 0; i < VIR_CGROUP_CONTROLLER_LAST; i++) {
+            VIR_DEBUG("Controller '%s' wanted=%s, mount='%s'",
+                      virCgroupControllerTypeToString(i),
+                      (1 << i) & controllers ? "yes" : "no",
+                      NULLSTR(group->controllers[i].mountPoint));
+            if (!((1 << i) & controllers) &&
+                group->controllers[i].mountPoint) {
                 /* Check whether a request to disable a controller
                  * clashes with co-mounting of controllers */
                 for (j = 0; j < VIR_CGROUP_CONTROLLER_LAST; j++) {