Add missing cancel_op(currently being read op) when stopping a client

Fixes client hangs possible after stopping & restarting an osd.
Hangs happened when a connection was closed in the middle of reading a READ
operation reply from the network. In this case the operation being read was
in read_op and the client didn't free it when closing the connection.

Test case for msgr_read.cpp:
- Partially read reply for a READ operation
- stop_client()
- Check that the READ operation returns EPIPE

The bug was actually introduced in 0.5.11.

src/msgr_stop.cpp

@@ -111,6 +111,10 @@ void osd_messenger_t::stop_client(int peer_fd, bool force, bool force_delete)
         {
             delete cl->read_op;
         }
+        else
+        {
+            cancel_op(cl->read_op);
+        }
         cl->read_op = NULL;
     }
     if (cl->osd_num)