ntfs: update super block operations

Update the super block operations to support the new fs_context-based
mount API, full read-write support including ->sync_fs, and file system
shutdown support.

Update ntfs_statfs() to provide statistics using atomic counters for free
clusters and MFT records.

Add a dedicated workqueue to compute the total number of free clusters by
scanning  asynchronously. Synchronous bitmap scanning during mount
can take a very long time on large volumes, severely delaying mount
completion. Moving this to the background allows mount to finish almost
immediately.

Implement ntfs_write_volume_label() to allow changing the volume label.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>