s390: add kmemleak annotation in stack_alloc()

kmemleak with enabled auto scanning reports that our stack allocation is
lost. This is because we're saving the pointer + STACK_INIT_OFFSET to
lowcore. When kmemleak now scans the objects, it thinks that this one is
lost because it can't find a corresponding pointer.
Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Tested-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/setup.c

@@ -50,6 +50,7 @@
 #include <linux/compat.h>
 #include <linux/start_kernel.h>
 #include <linux/hugetlb.h>
+#include <linux/kmemleak.h>
 
 #include <asm/boot_data.h>
 #include <asm/ipl.h>
@@ -356,9 +357,12 @@ void *restart_stack;
 unsigned long stack_alloc(void)
 {
 #ifdef CONFIG_VMAP_STACK
-	return (unsigned long)__vmalloc_node(THREAD_SIZE, THREAD_SIZE,
-			THREADINFO_GFP, NUMA_NO_NODE,
-			__builtin_return_address(0));
+	void *ret;
+
+	ret = __vmalloc_node(THREAD_SIZE, THREAD_SIZE, THREADINFO_GFP,
+			     NUMA_NO_NODE, __builtin_return_address(0));
+	kmemleak_not_leak(ret);
+	return (unsigned long)ret;
 #else
 	return __get_free_pages(GFP_KERNEL, THREAD_SIZE_ORDER);
 #endif