exec: Remove BUG_ON(has_group_leader_pid)

With the introduction of exchange_tids thread_group_leader and
has_group_leader_pid have become equivalent.  Further at this point in the
code a thread group has exactly two threads, the previous thread_group_leader
that is waiting to be reaped and tsk.  So we know it is impossible for tsk to
be the thread_group_leader.

This is also the last user of has_group_leader_pid so removing this check
will allow has_group_leader_pid to be removed.

So remove the "BUG_ON(has_group_leader_pid)" that will never fire.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/exec.c

@@ -1176,7 +1176,6 @@ static int de_thread(struct task_struct *tsk)
 		tsk->start_boottime = leader->start_boottime;
 
 		BUG_ON(!same_thread_group(leader, tsk));
-		BUG_ON(has_group_leader_pid(tsk));
 		/*
 		 * An exec() starts a new thread group with the
 		 * TGID of the previous thread group. Rehash the