restore: die properly if restore_one_task fails

This is really just the last bit of c32046c9; if restore_one_task() fails, we
need to do the same futex wakeup we do everywhere else in this function.

v2: use err instead of err_fini_mnt after mount has been finalized normally

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>