hash: New helper functions hash_bytes32() and hash_bytes64().

All of the callers of hash_words() and hash_words64() actually find it
easier to pass in the number of bytes instead of the number of 32-bit
or 64-bit words.  These new functions allow the callers to be a little
simpler.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>