Missing arrayOffset in FBUtilities.hash

In CASSANDRA-3869, FBUtilities.hash was optimised to use the backing byte array if there is one. However, there is a missing +arrayOffset in the offset parameter.

This can cause incorrect hashes resulting in data going to the wrong place, etc.. I haven't observed any errors directly attributable to this so maybe we are lucky and all backing arrays start at 0 but this could cause data loss in the worst case.