hash: Change mhash_finish() data measurement from words to bytes.

hash: Change mhash_finish() data measurement from words to bytes.

murmurhash includes an xor with the number of bytes hashed in its finishing
step.  Until now, we've only had murmurhash for full words, but an upcoming
commit adds murmurhash for bytes, so the finishing function will need to
take a count of bytes instead.

Signed-off-by: Ben Pfaff <blp@nicira.com>