xfs: use a struct iomap in xfs_writepage_ctx

xfs: use a struct iomap in xfs_writepage_ctx

Source kernel commit: 4e087a3b313cc664057279b1e40372fc97e3e212

In preparation for moving the XFS writeback code to fs/iomap.c, switch
it to use struct iomap instead of the XFS-specific struct xfs_bmbt_irec.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>