Convert XFS to use the new iomap based multipage write path. This involves implementing the ->iomap_begin and ->iomap_end methods, and switching the buffered file write, page_mkwrite and xfs_iozero paths to the new iomap helpers. With this change __xfs_get_blocks will never be used for buffered writes, and the code handling them can be removed. Based on earlier code from Dave Chinner. Signed-off-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Bob Peterson <rpeterso@redhat.com> Signed-off-by: Dave Chinner <david@fromorbit.com>