sched/mmcid: Prevent CID stalls due to concurrent forks

In the Linux kernel, the following vulnerability has been resolved:

sched/mmcid: Prevent CID stalls due to concurrent forks

A newly forked task is accounted as MMCID user before the task is visible
in the process' thread list and the global task list. This creates the
following problem:

CPU1 CPU2
fork()
sched_mm_cid_fork(tnew1)
tnew1->mm.mm_cid_users++;
tnew1->mm_cid.cid = getcid()
-> preemption
fork()
sched_mm_cid_fork(tnew2)
tnew2->mm.mm_cid_users++;
// Reaches the per CPU threshold
mm_cid_fixup_tasks_to_cpus()
for_each_other(current, p)
....

As tnew1 is not visible yet, this fails to fix up the already allocated CID
of tnew1. As a consequence a subsequent schedule in might fail to acquire a
(transitional) CID and the machine stalls.

Move the invocation of sched_mm_cid_fork() after the new task becomes
visible in the thread and the task list to prevent this.

This also makes it symmetrical vs. exit() where the task is removed as CID
user before the task is removed from the thread and task lists.