xfs: clean up printing inode32/64 in xfs_showargs

inode64 is the only value remaining in the unset array.  Special case
the inode32/64 options with an explicit seq_printf that prints either
inode32 or inode64, and remove the unset array.

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