Only attempt file unlock if we use locking

Fixes a segfault on exit, if file locking isn't used.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/ioengines.c b/ioengines.c
index 234f8ed..4d1491c 100644
--- a/ioengines.c
+++ b/ioengines.c
@@ -475,7 +475,9 @@
 	fio_file_set_closing(f);
 
 	disk_util_dec(f->du);
-	unlock_file_all(td, f);
+
+	if (td->o.file_lock_mode != FILE_LOCK_NONE)
+		unlock_file_all(td, f);
 
 	return put_file(td, f);
 }