null: add FIO_FAKEIO flag

Tell the backend that null just fakes IO, and use this knowledge
to have verifies automagically work when using null. This allows
testing of a wider range of paths with ioengine=null.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/verify.c b/verify.c
index 217b686..3d7c5de 100644
--- a/verify.c
+++ b/verify.c
@@ -759,6 +759,13 @@
 
 	if (td->o.verify == VERIFY_NULL || io_u->ddir != DDIR_READ)
 		return 0;
+	/*
+	 * If the IO engine is faking IO (like null), then just pretend
+	 * we verified everything.
+	 */
+	if (td->io_ops->flags & FIO_FAKEIO)
+		return 0;
+
 	if (io_u->flags & IO_U_F_TRIMMED) {
 		ret = verify_trimmed_io_u(td, io_u);
 		goto done;