xfs: support attrfork and unwritten BUIs

In preparation for atomic extent swapping and the online repair
functionality that wants atomic extent swaps, enhance the BUI code so
that we can support deferred work on the extended attribute fork and on
unwritten extents.
-----BEGIN PGP SIGNATURE-----
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=uJ2I
-----END PGP SIGNATURE-----