xfs: set the scrub AG number in xchk_ag_read_headers

Since xchk_ag_read_headers initializes fields in struct xchk_ag, we
might as well set the AG number and save the callers the trouble.

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