mm: set task exit code before complete_vfork_done()

mm: set task exit code before complete_vfork_done()

kthread_stop() uses task->vfork_done for synchronization.  The exiting
kthread shouldn't do complete_vfork_done() until it sets ->exit_code.

fix for mm-correctly-synchronize-rss-counters-at-exit-exec.patch

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>