bcachefs fixes for 6.10-rc4

- fix kworker explosion, due to calling submit_bio() (which can block)
  from a multithreaded workqueue
- fix error handling in btree node scan
- forward compat fix: kill an old debug assert
- key cache shrinker fixes
  this is a partial fix for stalls doing multithreaded creates - there
  were various O(n^2) issues the key cache shrinker was hitting

  https://lore.kernel.org/linux-bcachefs/fmfpgkt3dlzxhotrfmqg3j3wn5bpqiqvlc44xllncvdkimmx3i@n4okabtvhu7t/

  there's more work coming here; I'm working on a patch to delete the
  key cache lock, which initial testing shows to be a pretty drastic
  performance improvement
- assorted syzbot fixes
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