Add 'sequential' file_service_type

This service type will keep a file open until IO to it is completely
done, before moving on to the next available file.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/options.c b/options.c
index fe890df..cec7bb7 100644
--- a/options.c
+++ b/options.c
@@ -871,6 +871,10 @@
 			    .oval = FIO_FSERVICE_RR,
 			    .help = "Round robin select files",
 			  },
+			  { .ival = "sequential",
+			    .oval = FIO_FSERVICE_SEQ,
+			    .help = "Finish one file before moving to the next",
+			  },
 		},
 		.parent = "nrfiles",
 	},