ceph: limit osd read size to CEPH_MSG_MAX_DATA_LEN

libceph returns -EIO when read size > CEPH_MSG_MAX_DATA_LEN.

Link: http://tracker.ceph.com/issues/20528
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
diff --git a/fs/ceph/super.h b/fs/ceph/super.h
index da036d0..2b1b021 100644
--- a/fs/ceph/super.h
+++ b/fs/ceph/super.h
@@ -46,7 +46,8 @@
 #define ceph_test_mount_opt(fsc, opt) \
 	(!!((fsc)->mount_options->flags & CEPH_MOUNT_OPT_##opt))
 
-#define CEPH_RSIZE_DEFAULT              (64*1024*1024) /* max read size */
+/* max size of osd read request, limited by libceph */
+#define CEPH_MAX_READ_SIZE              CEPH_MSG_MAX_DATA_LEN
 #define CEPH_RASIZE_DEFAULT             (8192*1024)    /* max readahead */
 #define CEPH_MAX_READDIR_DEFAULT        1024
 #define CEPH_MAX_READDIR_BYTES_DEFAULT  (512*1024)