xfs: fix superfluous clearing of info->low in __xfs_getfsmap_datadev

xfs: fix superfluous clearing of info->low in __xfs_getfsmap_datadev

The for_each_perag helpers update the agno passed in for each iteration,
and thus the "if (pag->pag_agno == start_ag)" check will always be true.

Add another variable for the loop iterator so that the field is only
cleared after the first iteration.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>