rbd: Clear ceph_msg->bio_iter for retransmitted message

The bug can cause NULL pointer dereference in write_partial_msg_pages
Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
Reviewed-by: Alex Elder <elder@inktank.com>

net/ceph/messenger.c

@@ -649,6 +649,10 @@ static void prepare_write_message(struct ceph_connection *con)
 		m->hdr.seq = cpu_to_le64(++con->out_seq);
 		m->needs_out_seq = false;
 	}
+#ifdef CONFIG_BLOCK
+	else
+		m->bio_iter = NULL;
+#endif
 
 	dout("prepare_write_message %p seq %lld type %d len %d+%d+%d %d pgs\n",
 	     m, con->out_seq, le16_to_cpu(m->hdr.type),