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/ioengine.h b/ioengine.h
index 29c8487..ebe0ebe 100644
--- a/ioengine.h
+++ b/ioengine.h
@@ -166,6 +166,7 @@
 	FIO_BARRIER	= 1 << 8,	/* engine supports barriers */
 	FIO_MEMALIGN	= 1 << 9,	/* engine wants aligned memory */
 	FIO_BIT_BASED	= 1 << 10,	/* engine uses a bit base (e.g. uses Kbit as opposed to KB) */
+	FIO_FAKEIO	= 1 << 11,	/* engine pretends to do IO */
 };
 
 /*