blk-bench: don't write if not requested

Because a check for the TEST_WRITE configuration was missing as condition for a
packet submit, the test caused unwanted write access to the device.

Ref #1824

repos/os/src/test/blk/bench/main.cc

@@ -46,6 +46,9 @@ class Test::Throughput
 		{
 			static size_t count            = REQUEST_SIZE / _blk_size;
 
+			if (_read_done && (_write_done || !TEST_WRITE))
+				return;
+
 			try {
 				while (_session.tx()->ready_to_submit()) {
 					Block::Packet_descriptor p(