RDS/IB: track signaled sends

We're seeing bugs today where IB connection shutdown clears the send
ring while the tasklet is processing completed sends.  Implementation
details cause this to dereference a null pointer.  Shutdown needs to
wait for send completion to stop before tearing down the connection.  We
can't simply wait for the ring to empty because it may contain
unsignaled sends that will never be processed.

This patch tracks the number of signaled sends that we've posted and
waits for them to complete.  It also makes sure that the tasklet has
finished executing.

Signed-off-by: Zach Brown <zach.brown@oracle.com>

net/rds/ib.h

@@ -108,6 +108,7 @@ struct rds_ib_connection {
 	struct rds_header	*i_send_hdrs;
 	u64			i_send_hdrs_dma;
 	struct rds_ib_send_work *i_sends;
+	atomic_t		i_signaled_sends;
 
 	/* rx */
 	struct tasklet_struct	i_recv_tasklet;