test: sigaltstack -- Setup proper stack size

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>

test/zdtm/static/sigaltstack.c

@@ -64,7 +64,7 @@ void thread_sigaction(int signo, siginfo_t *info, void *context)
 static void *thread_func(void *arg)
 {
 	sas_state[SAS_THRD_OLD] = (stack_t) {
-		.ss_size	= SIGSTKSZ,
+		.ss_size	= sizeof(stack_thread) - 8,
 		.ss_sp		= stack_thread,
 		.ss_flags	= SS_ONSTACK,
 	};
@@ -106,7 +106,7 @@ int main(int argc, char *argv[])
 	pthread_t thread;
 
 	sas_state[SAS_MAIN_OLD] = (stack_t) {
-		.ss_size	= SIGSTKSZ,
+		.ss_size	= sizeof(stack_main) - 8,
 		.ss_sp		= stack_main,
 		.ss_flags	= SS_ONSTACK,
 	};