The existing hash algorithm in bufhash() does not distribute entries

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.

Merged from FreeBSD