crush_hash_seed
__u32 hash = crush_hash_seed ^ a;
__u32 hash = crush_hash_seed ^ a ^ b;
__u32 hash = crush_hash_seed ^ a ^ b ^ c;
__u32 hash = crush_hash_seed ^ a ^ b ^ c ^ d;
__u32 hash = crush_hash_seed ^ a ^ b ^ c ^ d ^ e;