s390/protvirt: support ultravisor without secure storage limit

Avoid potential crash due to lack of secure storage limit. Check that
max_sec_stor_addr is not 0 before adjusting vmalloc position.
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/uv.c

@@ -90,6 +90,7 @@ void __init setup_uv(void)
 
 void adjust_to_uv_max(unsigned long *vmax)
 {
+	if (uv_info.max_sec_stor_addr)
 		*vmax = min_t(unsigned long, *vmax, uv_info.max_sec_stor_addr);
 }