Initialize resid variable

Properly initialize the variable resid for the error case where
vn_rdwr() returns an error with setting *resid.  In practice
this is harmless because the contents of resid don't matter
when vn_rdwr() returns an error.  But the code does check the
result which means we may or may not end up having the check
zio->io_error.  This makes gcc unhappy and having a completely
deterministic code path here is a good thing.

Closes #51

module/zfs/vdev_file.c

@@ -143,7 +143,7 @@ vdev_file_io_start(zio_t *zio)
 {
 	vdev_t *vd = zio->io_vd;
 	vdev_file_t *vf = vd->vdev_tsd;
-	ssize_t resid;
+	ssize_t resid = 0;
 
 	if (zio->io_type == ZIO_TYPE_IOCTL) {
 		/* XXPOLICY */