xfs: turn io_append_trans into an io_private void pointer

In preparation for moving the ioend structure to common code we need
to get rid of the xfs-specific xfs_trans type.  Just make it a file
system private void pointer instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>