bcachefs fixes for 6.16-rc2

As usual, highlighting the ones users have been noticing:

- Fix a small issue with has_case_insensitive not being propagated on
  snapshot creation; this led to fsck errors, which we're harmless
  because we're not using this flag yet (it's for overlayfs +
  casefolding).

- Log the error being corrected in the journal when we're doing fsck
  repair: this was one of the "lessons learned" from the i_nlink 0 ->
  subvolume deletion bug, where reconstructing what had happened by
  analyzing the journal was a bit more difficult than it needed to be.

- Don't schedule btree node scan to run in the superblock: this fixes a
  regression from the 6.16 recovery passes rework, and let to it running
  unnecessarily.

  The real issue here is that we don't have online, "self healing" style
  topology repair yet: topology repair currently has to run before we go
  RW, which means that we may schedule it unnecessarily after a
  transient error. This will be fixed in the future.

- We now track, in btree node flags, the reason it was scheduled to be
  rewritten. We discovered a deadlock in recovery when many btree nodes
  need to be rewritten because they're degraded: fully fixing this will
  take some work but it's now easier to see what's going on.

  For the bug report where this came up, a device had been kicked RO due
  to transient errors: manually setting it back to RW was sufficient to
  allow recovery to succeed.

- Mark a few more fsck errors as autofix: as a reminder to users, please
  do keep reporting cases where something needs to be repaired and is
  not repaired automatically (i.e. cases where -o fix_errors or fsck -y
  is required).

- rcu_pending.c now works with PREEMPT_RT

- 'bcachefs device add', then umount, then remount wasn't working - we
  now emit a uevent so that the new device's new superblock is correctly
  picked up

- Assorted repair fixes: btree node scan will no longer incorrectly
  update sb->version_min,

- Assorted syzbot fixes
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