ntfs3: refactor ntfs_writepages

Handle the resident case with an explicit generic_writepages call instead
of using the obscure overload that makes mpage_writepages with a NULL
get_block do the same thing.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>