unsigned long __get_nr_free_mft_records (ntfs_volume * vol, s64 nr_free, const pgoff_t max_index);
Calculate the number of free mft records (inodes) on the mounted NTFS volume vol. We actually calculate the number of mft records in use instead because this allows us to not care about partial pages as these will be just zero filled and hence not be counted as allocated mft record.
If any pages cannot be read we assume all mft records in the erroring pages are in use. This means we return an underestimate on errors which is better than an overestimate.
Caller must hold mftbmp_lock rw_semaphore for reading or writing.