[PATCH] ITIMER_REAL: fix possible deadlock and race

As Steven Rostedt pointed out, there are 2 problems with ITIMER_REAL
timers.

1. do_setitimer() does not call del_timer_sync() in case
   when the timer is not pending (it_real_value() returns 0).
   This is wrong, the timer may still be running, and it can
   rearm itself.

2. It calls del_timer_sync() with tsk->sighand->siglock held.
   This is deadlockable, because timer's handler needs this
   lock too.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>