lib: Don't use alloca over net received size

Better to obtain error if there is no free memory
than smashing the stack.

A rule of thumb for alloca() based functions is to
use them with predefined small sizes (such as we
do in swapping builtin sizes for parasite engine).

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>