Here you can find that HASH_LEN_DECLARE
is defined as:
/* The hash is always the low bits of hash_len */
#ifdef __LITTLE_ENDIAN
#define HASH_LEN_DECLARE u32 hash; u32 len;
#else
#define HASH_LEN_DECLARE u32 len; u32 hash;
#endif
if little-endian (machine stores the least significant byte first) hash
is the first member, else hash
is the last member.