don't log iopiece when do_verify is off

Don't log an iopiece if do_verify is off. This allows us to do large write
phases (with verify headers) without using a lot of memory.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index cab0980..93c451c 100644
--- a/io_u.c
+++ b/io_u.c
@@ -763,6 +763,7 @@
 		io_u_mark_latency(td, usec);
 
 		if (td_write(td) && idx == DDIR_WRITE &&
+		    td->o.do_verify &&
 		    td->o.verify != VERIFY_NONE)
 			log_io_piece(td, io_u);