Picked from patch "[PATCH RFC] namespaces: use CLONE_VFORK
with CLONE_VM when it is possible" by Andrew Vagin.

Currenly parent touches child's stack, as in moment of clone() call
its stack pointer is above the child's (we allocate char stack[128]
on parent's stack). This prevents to create CLONE_VM|CLONE_VFORK
processes, because the child uses stack addresses occupied by parent.

The patch changes clone_noasan() behaviour and allows to do that
with the same memory consumption. We give a child memory, which
is not used by parent clone(), so parent's and child's stacks
have no tntersection.

This allows to create CLONE_VM|CLONE_VFORK processes.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>