posix-cpu-timers: Extend rcu_read_lock removing task_struct references

Now that the code stores of pid references it is no longer necessary
or desirable to take a reference on task_struct in __get_task_for_clock.

Instead extend the scope of rcu_read_lock and remove the reference
counting on struct task_struct entirely.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>