[block] Ignore redundant xfer_window_changed() messages

For some block device protocols, the active path may continue to
receive xfer_window_changed() notifications during normal use.  These
currently result in the active path being erroneously closed.

Fix by ignoring any xfer_window_changed() messages if this path is
already the active path.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/sanboot.c

 static void sanpath_step ( struct san_path *sanpath ) {
 	struct san_device *sandev = sanpath->sandev;
 
+	/* Ignore if we are already the active device */
+	if ( sanpath == sandev->active )
+		return;
+
 	/* Wait until path has become available */
 	if ( ! xfer_window ( &sanpath->block ) )
 		return;