session02: hard-code the value of the macro CLONE_STACK_SIZE

The macro PAGE_SIZE is declared in the following way in Debian:

 #define PAGE_SIZE               (sysconf(_SC_PAGESIZE))

that makes it impossible to use the macro as the size of
the array clone_args::stack since this generates the following error:

cc -g -O2 -Wall -Werror -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0 -iquote ../../lib/arch/arm/include -I../../lib   session02.c ../../lib/libzdtmtst.a   -o session02
session02.c:70:7: error: variably modified 'stack' at file scope

Cc: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>