scoutfs: add hash helper

Add a quick header which calculates 64bit hashes by calculating the crc
of the two halves of a byte region.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/hash.h

@@ -0,0 +1,15 @@
+#ifndef _SCOUTFS_HASH_H_
+#define _SCOUTFS_HASH_H_
+
+#include <linux/crc32c.h>
+
+/* XXX replace with xxhash */
+static inline u64 scoutfs_hash64(const void *data, unsigned int len)
+{
+       unsigned int half = (len + 1) / 2;
+
+       return crc32c(~0, data, half) |
+              ((u64)crc32c(~0, data + len - half, half) << 32);
+}
+
+#endif