extend_file need continue when the file exists

If some testing files exist and some others don't exist, fio will
report failure on the no-exist file. Here is a patch to fix it.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/filesetup.c b/filesetup.c
index 785afce..d10840a 100644
--- a/filesetup.c
+++ b/filesetup.c
@@ -34,7 +34,7 @@
 		unlink_file = 1;
 
 	if (unlink_file || new_layout) {
-		if (unlink(f->file_name) < 0) {
+		if ((unlink(f->file_name) < 0) && (errno != ENOENT)) {
 			td_verror(td, errno, "unlink");
 			return 1;
 		}