kernel/ntfs - Fix a possible problem in bufcache.

- Avoid different block size bread() calls for the
  same blkno.
- Fix a bug in the code handling holes which could
  cause infinite loops.

Taken-from:    FreeBSD's r116620
kernel/ntfs - Fix buffer overlapping problems.

- Read only one cluster at a time.
- Do not cache boot block.

Taken-from:    FreeBSD's r118035
Dragonfly-bug: <http://bugs.dragonflybsd.org/issues/2388>

(Commit refs #2388 and fixes #1853)
kernel/ntfs - Remove a forgotten #if defined