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.

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

iQIzBAABCgAdFiEEUzaAxoMeQq6m2jMV+H93GTRKtOsFAmNoGzoACgkQ+H93GTRK
tOv5+RAAnfpapBakJ9vCCqduCkxKSPuXOyJrixg//m4TMGy3UjzlBXcnr6mmcDx8
nfeNhtX02gAoWxvIdtPzjlWMyOSfgzwAXWEIuKHoc8qoOL+E/ulTI5wb1BWAveI0
F0aEwI5DBfNDt749aBRRXj2RyfaQp6McFGYAUHCI2E+weiD097njSGVBO2bTR35K
T/5cWwD4v+O546igy05hT2e60/MIR5QUX+Bbsg0zTki9ChBkkfZMo7+wsCFOunej
CpKVC0WO3Qkq/9m9p/yfjuMqCiN2wqEtKs31g3qzn1eDlPdxafV2h2WwvHM4g1mI
tWHq/fZ/KJfK+z5WPQzw2z4YkAw5r2XfrWXyJOCXylM81mi5FN1w7i68Zlt6Zt1y
w4T/TPGlocXjYPkoLwVGi/QAkzXbZmOn7QgdDDJPj+51HUdWgkEzYnnnQJk1h8HP
z1zZS+RcTiqYDl3s9rPwlNADWHEJ9ozjaXyNv35DB33UAghTxfeNs7irmTR8ZeYk
zikjSl7uWl5ZPIe/RFe76i1j1xGt/B2BKUDcBujPWqK/WgiHHbDedxm8G+dXrSfC
6GnvjD07Vv24eGmT6tRITpySDfHJu59iw0lioibpXskrzrI4QufgfVB5gW2Uj6WS
100y0TTzZo5YsvOyCyZJO56IP0fsOU1mxWoiKYAgCCxOKQun2r8=
=ajnu
-----END PGP SIGNATURE-----