xen-blkfront: fix length check

Length is an unsigned integer, so checking against < 0 doesn't make
sense. While there also make clear that a length of 0 always succeeds.

Submitted by:		Pratyush Yadav <pratyush@freebsd.org>
Differential Review:	https://reviews.freebsd.org/D16045

sys/dev/xen/blkfront/blkfront.c

@@ -602,8 +602,8 @@ xbd_dump(void *arg, void *virtual, vm_offset_t physical, off_t offset,
 	int sbp;
 	int rc = 0;
 
-	if (length <= 0)
-		return (rc);
+	if (length == 0)
+		return (0);
 
 	xbd_quiesce(sc);	/* All quiet on the western front. */