xfs: remove the inode log format from the inode log item

No need to keep the inode log format around all the time, we can
easily generate it at iop_format time.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>