siphash: use one source of truth for siphash permutations

The SipHash family of permutations is currently used in three places:

- siphash.c itself, used in the ordinary way it was intended.
- random32.c, in a construction from an anonymous contributor.
- random.c, as part of its fast_mix function.

Each one of these places reinvents the wheel with the same C code, same
rotation constants, and same symmetry-breaking constants.

This commit tidies things up a bit by placing macros for the
permutations and constants into siphash.h, where each of the three .c
users can access them. It also leaves a note dissuading more users of
them from emerging.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

include/linux/siphash.h

@@ -138,4 +138,32 @@ static inline u32 hsiphash(const void *data, size_t len,
 	return ___hsiphash_aligned(data, len, key);
 }
 
+/*
+ * These macros expose the raw SipHash and HalfSipHash permutations.
+ * Do not use them directly! If you think you have a use for them,
+ * be sure to CC the maintainer of this file explaining why.
+ */
+
+#define SIPHASH_PERMUTATION(a, b, c, d) ( \
+	(a) += (b), (b) = rol64((b), 13), (b) ^= (a), (a) = rol64((a), 32), \
+	(c) += (d), (d) = rol64((d), 16), (d) ^= (c), \
+	(a) += (d), (d) = rol64((d), 21), (d) ^= (a), \
+	(c) += (b), (b) = rol64((b), 17), (b) ^= (c), (c) = rol64((c), 32))
+
+#define SIPHASH_CONST_0 0x736f6d6570736575ULL
+#define SIPHASH_CONST_1 0x646f72616e646f6dULL
+#define SIPHASH_CONST_2 0x6c7967656e657261ULL
+#define SIPHASH_CONST_3 0x7465646279746573ULL
+
+#define HSIPHASH_PERMUTATION(a, b, c, d) ( \
+	(a) += (b), (b) = rol32((b), 5), (b) ^= (a), (a) = rol32((a), 16), \
+	(c) += (d), (d) = rol32((d), 8), (d) ^= (c), \
+	(a) += (d), (d) = rol32((d), 7), (d) ^= (a), \
+	(c) += (b), (b) = rol32((b), 13), (b) ^= (c), (c) = rol32((c), 16))
+
+#define HSIPHASH_CONST_0 0U
+#define HSIPHASH_CONST_1 0U
+#define HSIPHASH_CONST_2 0x6c796765U
+#define HSIPHASH_CONST_3 0x74656462U
+
 #endif /* _LINUX_SIPHASH_H */