libceph: don't call ->reencode_message() more than once per message

Reencoding an already reencoded message is a bad idea.  This could
happen on Policy::stateful_server connections (!CEPH_MSG_CONNECT_LOSSY),
such as MDS sessions.

This didn't pop up in testing because currently only OSD requests are
reencoded and OSD sessions are always lossy.

Fixes: 98ad5ebd ("libceph: ceph_connection_operations::reencode_message() method")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>

net/ceph/messenger.c

@@ -1287,10 +1287,10 @@ static void prepare_write_message(struct ceph_connection *con)
 	if (m->needs_out_seq) {
 		m->hdr.seq = cpu_to_le64(++con->out_seq);
 		m->needs_out_seq = false;
-	}
 
-	if (con->ops->reencode_message)
-		con->ops->reencode_message(m);
+		if (con->ops->reencode_message)
+			con->ops->reencode_message(m);
+	}
 
 	dout("prepare_write_message %p seq %lld type %d len %d+%d+%zd\n",
 	     m, con->out_seq, le16_to_cpu(m->hdr.type),