xfs: remove unused struct xfs_mount field m_fsname_len

The struct xfs_mount field m_fsname_len is not used anywhere, remove it.

Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-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>