kernel/exit.c: Use list_for_each_entry(_safe) instead of list_for_each(_safe)

kernel/exit.c: Convert list_for_each(_safe) to
list_for_each_entry(_safe) in forget_original_parent(), exit_notify()
and do_wait()

Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>