bcachefs repair code for 6.9-rc3

A couple more small fixes, and new repair code.

We can now automatically recover from arbitrary corrupted interior btree
nodes by scanning, and we can reconstruct metadata as needed to bring a
filesystem back into a working, consistent, read-write state and
preserve access to whatevver wasn't corrupted.

Meaning - you can blow away all metadata except for extents and dirents
leaf nodes, and repair will reconstruct everything else and give you
your data, and under the correct paths. If inodes are missing i_size
will be slightly off and permissions/ownership/timestamps will be gone,
and we do still need the snapshots btree if snapshots were in use - in
the future we'll be able to guess the snapshot tree structure in some
situations.

IOW - aside from shaking out remaining bugs (fuzz testing is still
coming), repair code should be complete and if repair ever doesn't work
that's the highest priority bug that I want to know about immediately.

This patchset was kindly tested by a user from India who accidentally
wiped one drive out of a three drive filesystem with no replication on
the family computer - it took a couple weeks but we got everything
important back.
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