libceph: bump CEPH_MSG_MAX_DATA_LEN (again)

This time for rbd object map.  Object maps are limited in size to
256000000 objects, two bits per object.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>

include/linux/ceph/libceph.h

@@ -84,11 +84,13 @@ struct ceph_options {
 #define CEPH_MSG_MAX_MIDDLE_LEN	(16*1024*1024)
 
 /*
- * Handle the largest possible rbd object in one message.
+ * The largest possible rbd data object is 32M.
+ * The largest possible rbd object map object is 64M.
+ *
  * There is no limit on the size of cephfs objects, but it has to obey
  * rsize and wsize mount options anyway.
  */
-#define CEPH_MSG_MAX_DATA_LEN	(32*1024*1024)
+#define CEPH_MSG_MAX_DATA_LEN	(64*1024*1024)
 
 #define CEPH_AUTH_NAME_DEFAULT   "guest"