Make bsd process accounting work in pid namespaces

After I fixed access to task->tgid in kernel/acct.c, Oleg
pointed out some bad side effects with this accounting vs
pid namespaces interaction.

So here is the approach to make this accounting work with
pid namespaces properly.

The idea is simple - when task dies it accounts itself in
each namespace it is visible from. That was the summary,
the details are in patches.

Signed-off-by: Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>