hpfs - Fix a couple panics and a little cleanup.

hpfs - Fix a couple panics and a little cleanup.

* Fix compilation with HPFS_DEBUG.

* Fix a panic due CNP_PDIRUNLOCK flag not being cleared.

* Fix a panic where returned vnode after a lookup is not
  NULL in the ENOENT case.

* Disable write support completely. It was pretty minimal
  and operations like create or rename were not supported.

It has been tested with a filesystem created by OS/2 Warp 2.1.
Copying data out of it worked fine, but there is still an
outstanding issue with overlapping buffers.