s390/boot: avoid potential amode31 truncation

Fixes: bb1520d5 ("s390/mm: start kernel with DAT enabled")
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/boot/startup.c

@@ -271,7 +271,7 @@ static void offset_vmlinux_info(unsigned long offset)
 static unsigned long reserve_amode31(unsigned long safe_addr)
 {
 	__amode31_base = PAGE_ALIGN(safe_addr);
-	return safe_addr + vmlinux.amode31_size;
+	return __amode31_base + vmlinux.amode31_size;
 }
 
 void startup_kernel(void)