tests/cgroup: use cgroup.kill in cg_killall()

If cgroup.kill file is supported make use of it.

Link: https://lore.kernel.org/r/[e-mail]
Cc: Tejun Heo <[e-mail]>
Cc: [e-mail]
Reviewed-by: Shakeel Butt <[e-mail]>
Acked-by: Roman Gushchin <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

tools/testing/selftests/cgroup/cgroup_util.c

···
 	char buf[PAGE_SIZE];
 	char *ptr = buf;
 
+	/* If cgroup.kill exists use it. */
+	if (!cg_write(cgroup, "cgroup.kill", "1"))
+		return 0;
+
 	if (cg_read(cgroup, "cgroup.procs", buf, sizeof(buf)))
 		return -1;