siphash24: introduce siphash24_compress_typesafe() macro

To prevent copy-and-paste mistake.

This also introduce in_addr_hash_func().

No functional change, just refactoring.

coccinelle/siphash24.cocci

@@ -0,0 +1,6 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+@@
+expression p, s;
+@@
+- siphash24_compress(&p, sizeof(p), s);
++ siphash24_compress_typesafe(p, s);