bcachefs fixes for 6.9-rc3

Lots of fixes for situations with extreme filesystem damage. One fix
("Fix journal pins in btree write buffer") applicable to normal usage;
also a dio performance fix.

New repair/construction code is in the final stages, should be ready in
about a week. Anyone that lost btree interior nodes (or a variety of
other damage) as a result of the splitbrain bug will be able to repair
then.
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