xfs: use super s_id instead of struct xfs_mount m_fsname

Eliminate struct xfs_mount field m_fsname by using the super block s_id
field directly.

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>