Fix numerous translation problems in msdosfs, related to the recent BUF/BIO

Fix numerous translation problems in msdosfs, related to the recent BUF/BIO
work.  The source code was using 'bn' and 'cn' and 'lbn' interchangeability
to mean any one of device block number, logical cluster number, or disk
cluster number (which is relative to some weird offset), and sometimes
overloaded the same variable name with different types of block numbers.
This resulted in a number of errors when the code was initially translated
to 64 bit offsets.

Reported-by: David Rhodus