[PATCH] reset real_timer target on exec leader change

When a noninitial thread does exec, it becomes the new group leader.  If
there is a ITIMER_REAL timer running, it points at the old group leader and
when it fires it can follow a stale pointer.  The timer data needs to be
reset to point at the exec'ing thread that is becoming the group leader.
This has to synchronize with any concurrent firing of the timer to make
sure that it_real_fn can never run when the data points to a thread that
might have been reaped already.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>