xfs: adjust the incore perag block_count when shrinking

If we reduce the number of blocks in an AG, we must update the incore
geometry values as well.

Fixes: 0800169e ("xfs: Pre-calculate per-AG agbno geometry")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>