Don't allow pre_read on IO engines that cannot seek

We cannot pre-read files, if we cannot seek back and read the same
data again. So just disable it with a warning to the user.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/engines/net.c b/engines/net.c
index 4ddacf2..c0a793e 100644
--- a/engines/net.c
+++ b/engines/net.c
@@ -633,7 +633,7 @@
 	.open_file	= fio_netio_open_file,
 	.close_file	= generic_close_file,
 	.flags		= FIO_SYNCIO | FIO_DISKLESSIO | FIO_UNIDIR |
-			  FIO_SIGQUIT,
+			  FIO_SIGQUIT | FIO_PIPEIO,
 };
 #endif
 
@@ -648,7 +648,7 @@
 	.open_file	= fio_netio_open_file,
 	.close_file	= fio_netio_close_file,
 	.flags		= FIO_SYNCIO | FIO_DISKLESSIO | FIO_UNIDIR |
-			  FIO_SIGQUIT,
+			  FIO_SIGQUIT | FIO_PIPEIO,
 };
 
 static void fio_init fio_netio_register(void)