xfs_repair: use libxfs function to calculate root inode location

xfs_repair: use libxfs function to calculate root inode location

Use libxfs_ialloc_calc_rootino to compute the location of the root
inode, and improve the function comments while we're at it.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>