libceph: generalize ceph_pg_mapping

In preparation for adding support for primary_temp mappings, generalize
struct ceph_pg_mapping so it can hold mappings other than pg_temp.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>

include/linux/ceph/osdmap.h

@@ -60,8 +60,13 @@ struct ceph_object_id {
 struct ceph_pg_mapping {
 	struct rb_node node;
 	struct ceph_pg pgid;
-	int len;
-	int osds[];
+
+	union {
+		struct {
+			int len;
+			int osds[];
+		} pg_temp;
+	};
 };
 
 struct ceph_osdmap {