A customer had a field problem with multi-threaded coredumps,
and I think the fix is the exit race (already fixed in 2.6), but I
also noticed a possible race between taking a coredump and fork().

If zap_threads() in fs/exec.c is called while a thread is in do_fork(),
but before the newly created thread is in the thread list, it is possible
to have a running thread during the coredump.  This is bad, but not
terribly bad.  However, in this situation it is also possible in
__exit_mm() that the final two threads check mm->core_waiters at
the same time and then both decrement mm->core_waiters, causing
it to go negative, and thus causing a BUG() in coredump_wait().

To fix this, I would like to propose the attached patch to 2.6.

Signed-off-by: Corey Minyard <minyard@acm.org>