bcachefs fixes for 6.14-rc2

- add a SubmittingPatches to clarify that patches submitted for bcachefs
  do, in fact, need to be tested
- discard path now correctly issues journal flushes when needed, this
  fixes performance issues when the filesystem is nearly full and we're
  bottlenecked on copygc
- fix a bug that could cause the pending rebalance work accounting to be
  off when devices are being onlined/offlined; users should report if
  they are still seeing this
- and a few more trivial ones
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