fat: use generic_getfssec(), fix generic_getfssec(), add dprintf

fat: use generic_getfssec(), fix generic_getfssec(), add dprintf

Use generic_getfssec() for the FAT filesystem.  Do a bunch of
calculations based on clusters rather than on sectors, so we don't
have to do the same thing N times for N sectors per cluster.

Fix boundary conditions in generic_getfssec().  Adjust iso9660 to
match the resulting interface change.

Add dprintf's to generic_getfssec() and a few other routines.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>