cgroup: handle ENODEV on cg_read_pid() gracefully

The recently added test case TEST-07-PID1.subgroup-kill.sh surfaced a
race: if we enumerate PIDs in a cgroup, and the cgroup is unlinked at
the very same time reading will result in ENODEV. We need to handle that
gracefully. Hence let's do so.

Noticed while looking at:

https://github.com/systemd/systemd/actions/runs/16143084441/job/45554929264?pr=38120

src/shared/cgroup-setup.c

@@ -367,6 +367,8 @@ int cg_migrate(
                         if (r < 0)
                                 return RET_GATHER(ret, r);
                 }
+                if (r == -ENODEV)
+                        continue;
                 if (r < 0)
                         return RET_GATHER(ret, r);
         } while (!done);

src/shared/cgroup-show.c

@@ -107,9 +107,12 @@ static int show_cgroup_one_by_path(
                 /* libvirt / qemu uses threaded mode and cgroup.procs cannot be read at the lower levels.
                  * From https://docs.kernel.org/admin-guide/cgroup-v2.html#threads,
                  * “cgroup.procs” in a threaded domain cgroup contains the PIDs of all processes in
-                 * the subtree and is not readable in the subtree proper. */
+                 * the subtree and is not readable in the subtree proper.
+                 *
+                 * ENODEV is generated when we enumerate processes from a cgroup and the cgroup is removed
+                 * concurrently. */
                 r = cg_read_pid(f, &pid, /* flags = */ 0);
-                if (IN_SET(r, 0, -EOPNOTSUPP))
+                if (IN_SET(r, 0, -EOPNOTSUPP, -ENODEV))
                         break;
                 if (r < 0)
                         return r;