zdtm: cgroup_ifpriomap: Improve skip check's robustness.

cgroup_ifpriomap test needs net_prio cgroup, which might not be
available. Make the .checkskip script check it.

Signed-off-by: Michał Mirosław <[email protected]>

Author: osctobe

test/zdtm/static/cgroup_ifpriomap.checkskip

@@ -1,3 +1,6 @@
 #!/bin/bash
+set -e
 
-! test -f /sys/fs/cgroup/cgroup.controllers
+test ! -f /sys/fs/cgroup/cgroup.controllers
+
+grep -q '^net_prio\s' /proc/cgroups