The existing hash algorithm in bufhash() does not distribute entries
very well across buckets, especially in the case of cylinder group blocks
which are located at a sequence of locations that are a multiple of a large
power of two apart. In the case of large file systems, one or possibly
a few of the hash chains can get excessively long. Replace the existing
hash algorithm with a variation on the Fibonacci hash.

Tested by: Ken Marx <kmarx@vicor.com>