We use test_msg helper inside alternative stack
which in turn allocates TEST_MSG_BUFFER_SIZE
by its own, so we increased the stack storage
in @stack_thread and @stack_main but miss to
inform the kernel about bigger stack size.

Thus in worst scenario we can screwup the stack
so provide proper size here.
Reported-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>