pi-futex: set PF_EXITING without taking ->pi_lock

It is a bit annoying that do_exit() takes ->pi_lock to set PF_EXITING.  All
we need is to synchronize with lookup_pi_state() which saw this task
without PF_EXITING under ->pi_lock.

Change do_exit() to use spin_unlock_wait().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>