xen: xenbus_probe_frontend: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 146562
Addresses-Coverity-ID: 146563
Signed-off-by: NGustavo A. R. Silva <garsilva@embeddedor.com>
Reviewed-by: NJuergen Gross <jgross@suse.com>
Signed-off-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/xenbus/xenbus_probe_frontend.c

@@ -379,10 +379,12 @@ static void xenbus_reset_frontend(char *fe, char *be, int be_state)
 	case XenbusStateConnected:
 		xenbus_printf(XBT_NIL, fe, "state", "%d", XenbusStateClosing);
 		xenbus_reset_wait_for_backend(be, XenbusStateClosing);
+		/* fall through */
 
 	case XenbusStateClosing:
 		xenbus_printf(XBT_NIL, fe, "state", "%d", XenbusStateClosed);
 		xenbus_reset_wait_for_backend(be, XenbusStateClosed);
+		/* fall through */
 
 	case XenbusStateClosed:
 		xenbus_printf(XBT_NIL, fe, "state", "%d", XenbusStateInitialising);