Revert "cgroup-util: Don't try to open pidfd for kernel threads"

The kernel patch was reverted so let's try again to open pidfds
for kernel threads.

This reverts commit ead48ec.

(cherry picked from commit 1ce69e0)

src/basic/cgroup-util.c

@@ -149,9 +149,7 @@ int cg_read_pidref(FILE *f, PidRef *ret, CGroupFlags flags) {
                 if (pid == 0)
                         return -EREMOTE;
 
-                /* We might read kernel thread pids from cgroup.procs for which we cannot create a pidfd so
-                 * catch those and don't try to create a pidfd for them. */
-                if (FLAGS_SET(flags, CGROUP_NO_PIDFD) || pid_is_kernel_thread(pid) > 0) {
+                if (FLAGS_SET(flags, CGROUP_NO_PIDFD)) {
                         *ret = PIDREF_MAKE_FROM_PID(pid);
                         return 1;
                 }