We now have two places where we if/elseif/elseif/else a vma
and take actions -- the prepare_mm_pid and open_vmas. I propose
to make this if/... onlce once (while preparing) and set up
a callback on vma to be called later (while opening).

This requires my patch that doesn't set -1 in alloc_vma or
Andrey's patch that does the same.
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>