NTFS: 2.0.24 - Cleanups.

NTFS: 2.0.24 - Cleanups.
- Treat BUG_ON() as ASSERT() not VERIFY(), i.e. do not use side effects
  inside BUG_ON(). (Adam J. Richter)
- Split logical OR expressions inside BUG_ON() into individual BUG_ON()
  calls for improved debugging. (Adam J. Richter)
- Add errors flag to the ntfs volume state, accessed via
  NVol{,Set,Clear}Errors(vol).
- Do not allow read-write remounts of read-only volumes with errors.
- Clarify comment for ntfs file operation sendfile which was added by
  Christoph Hellwig a while ago (just using generic_file_sendfile())
  to say that ntfs ->sendfile is only used for the case where the
  source data is on the ntfs partition and the destination is
  somewhere else, i.e. nothing we need to concern ourselves with.