bcachefs fixes for 6.15-rc1

More notable fixes:

- Fix for striping behaviour on tiering filesystems where replicas
  exceeds durability on destination target
- Fix a race in device removal where deleting alloc info races with the
  discard worker
- Some small stack usage improvements: this is just enough for KMSAN
  builds to not blow the stack, more is queued up for 6.16.
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