ntfs3: Call ntfs_write_begin() and ntfs_write_end() directly

There is only one kind of write_begin/write_end aops, so we don't need to
look up which aop it is, just make ntfs_write_begin() and ntfs_write_end()
available to this file and call them directly.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>