[PATCH] fix non-leader exec under ptrace

This reverts most of commit 30e0fca6.
It broke the case of non-leader MT exec when ptraced.
I think the bug it was intended to fix was already addressed by commit
788e05a6

.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>