xfs: enhance fs summary counter scrubber

This series makes two changes to the fs summary counter scrubber: first,
we should mark the scrub incomplete when we can't read the AG headers.
Second, it fixes a functionality gap where we don't actually check the
free rt extent count.

v23.2: fix pointless inline

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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