ceph: specify max_bytes on readdir replies

Specify max bytes in request to bound size of reply.  Add associated
mount option with default value of 512 KB.

Signed-off-by: Sage Weil <sage@newdream.net>
diff --git a/fs/ceph/ceph_fs.h b/fs/ceph/ceph_fs.h
index 715e39c..3b9eeed 100644
--- a/fs/ceph/ceph_fs.h
+++ b/fs/ceph/ceph_fs.h
@@ -361,6 +361,7 @@
 	struct {
 		__le32 frag;                 /* which dir fragment */
 		__le32 max_entries;          /* how many dentries to grab */
+		__le32 max_bytes;
 	} __attribute__ ((packed)) readdir;
 	struct {
 		__le32 mode;