s390/mm: add __rcu annotations

Add __rcu annotations so sparse correctly warns only if "slot" gets
derefenced without using rcu_dereference(). Right now we get warnings
because of the missing annotation:

arch/s390/mm/gmap.c:135:17: warning: incorrect type in assignment (different address spaces)
arch/s390/mm/gmap.c:135:17:    expected void **slot
arch/s390/mm/gmap.c:135:17:    got void [noderef] <asn:4>**
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/gmap.c

@@ -125,7 +125,7 @@ static void gmap_radix_tree_free(struct radix_tree_root *root)
 	struct radix_tree_iter iter;
 	unsigned long indices[16];
 	unsigned long index;
-	void **slot;
+	void __rcu **slot;
 	int i, nr;
 
 	/* A radix tree is freed by deleting all of its entries */
@@ -150,7 +150,7 @@ static void gmap_rmap_radix_tree_free(struct radix_tree_root *root)
 	struct radix_tree_iter iter;
 	unsigned long indices[16];
 	unsigned long index;
-	void **slot;
+	void __rcu **slot;
 	int i, nr;
 
 	/* A radix tree is freed by deleting all of its entries */
@@ -1011,7 +1011,7 @@ EXPORT_SYMBOL_GPL(gmap_read_table);
 static inline void gmap_insert_rmap(struct gmap *sg, unsigned long vmaddr,
 				    struct gmap_rmap *rmap)
 {
-	void **slot;
+	void __rcu **slot;
 
 	BUG_ON(!gmap_is_shadow(sg));
 	slot = radix_tree_lookup_slot(&sg->host_to_rmap, vmaddr >> PAGE_SHIFT);