Args for buf_track() might be unused

If neither FULL_BUF_TRACKING nor BUF_TRACKING are defined, then the body of
buf_track() becomes empty. Mark the arguments with "__unused" so the
compiler doesn't complain about unused arguments in that case.

Reported by: Bruce Leverett (Panasas)
Reviewed by: cem (on IRC)
MFC after: 1 month
Sponsored by: Panasas