posix-timers: simplify de_thread()->exit_itimers() path

Impact: simplify code

de_thread() postpones release_task(leader) until after exit_itimers().
This was needed because !SIGEV_THREAD_ID timers could use ->group_leader
without get_task_struct(). With the recent changes we can release the
leader earlier and simplify the code.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>