posix-cpu-timers: Replace __get_task_for_clock with pid_for_clock

Now that the codes store references to pids instead of referendes to
tasks.  Looking up a task for a clock instead of looking up a struct
pid makes the code more difficult to verify it is correct than
necessary.

In posix_cpu_timers_create get_task_pid can race with release_task for
threads and return a NULL pid.  As put_pid and cpu_timer_task_rcu
handle NULL pids just fine the code works without problems but it is
an extra case to consider and keep in mind while verifying and
modifying the code.

There are races with de_thread to consider that only don't apply
because thread clocks are only allowed for threads in the same
thread_group.

So instead of leaving a burden for people making modification to the
code in the future return a rcu protected struct pid for the clock
instead.

The logic for __get_task_for_pid and lookup_task has been folded into
the new function pid_for_clock with the only change being the logic
has been modified from working on a task to working on a pid that
will be returned.

In posix_cpu_clock_get instead of calling pid_for_clock checking the
result and then calling pid_task to get the task.  The result of
pid_for_clock is fed directly into pid_task.  This is safe because
pid_task handles NULL pids.  As such an extra error check was
unnecessary.

Instead of hiding the flag that enables the special clock_gettime
handling, I have made the 3 callers just pass the flag in themselves.
That is less code and seems just as simple to work with as the
wrapper functions.

Historically the clock_gettime special case of allowing a process
clock to be found by the thread id did not even exist [33ab0fec]
but Thomas Gleixner reports that he has found code that uses that
functionality [55e8c8eb].

Link: https://lkml.kernel.org/r/87zhaxqkwa.fsf@nanos.tec.linutronix.de/
Ref: 33ab0fec ("posix-timers: Consolidate posix_cpu_clock_get()")
Ref: 55e8c8eb

 ("posix-cpu-timers: Store a reference to a pid not a task")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>