fuse: switch from DFLTPHYS/MAXBSIZE to maxcachebuf

On GENERIC kernels with empty loader.conf, there is no functional change.
DFLTPHYS and MAXBSIZE are both 64kB at the moment. This change allows
larger bufcache block sizes to be used when either MAXBSIZE (custom kernel)
or the loader.conf tunable vfs.maxbcachebuf (GENERIC) is adjusted higher
than the default.

Suggested by: ken@