bcachefs fixes for 6.9-rc1

Assorted bugfixes.

Most are fixes for simple assertion pops; the most significant fix is
for a deadlock in recovery when we have to rewrite large numbers of
btree nodes to fix errors. This was incorrectly running out of the same
workqueue as the core interior btree update path - we now give it its
own single threaded workqueue.

This was visible to users as "bch2_btree_update_start(): error:
BCH_ERR_journal_reclaim_would_deadlock" - and then recovery hanging.
-----BEGIN PGP SIGNATURE-----
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=J/hu
-----END PGP SIGNATURE-----