bcachefs fixes for 6.11-rc8

- fix ca->io_ref usage; analagous to previous patch doing that for main
  discard path
- cond_resched() in __journal_keys_sort(), cutting down on "hung task"
  warnings when journal is big
- rest of basic BCH_SB_MEMBER_INVALID support
- and the critical one: don't delete open files in online fsck, this was
  causing the "dirent points to inode that doesn't point back"
  inconsistencies some users were seeing
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