input/seat: simplify seat_is_input_allowed()

Use an early return to make the code more readable.

sway/input/seat.c

@@ -1092,7 +1092,9 @@ void seat_configure_xcursor(struct sway_seat *seat) {
 
 bool seat_is_input_allowed(struct sway_seat *seat,
 		struct wlr_surface *surface) {
-	if (server.session_lock.locked) {
+	if (!server.session_lock.locked) {
+		return true;
+	}
 	if (server.session_lock.lock == NULL) {
 		return false;
 	}
@@ -1103,8 +1105,6 @@ bool seat_is_input_allowed(struct sway_seat *seat,
 		}
 	}
 	return false;
-	}
-	return true;
 }
 
 static void send_unfocus(struct sway_container *con, void *data) {