xfs: move the di_flushiter field to struct xfs_inode

In preparation of removing the historic icinode struct, move the
flushiter field into the containing xfs_inode structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>