bcachefs fixes for 6.15-rc6

- Some fixes to help with filesystem analysis: ensure superblock error
  count gets written if we go ERO, don't discard the journal
  aggressively (so it's available for list_journal -a).

- Fix lost wakeup on arm causing us to get stuck when reading btree
  nodes.

- Fix fsck failing to exit on ctrl-c.

- An additional fix for filesystems with misaligned bucket sizes: we now
  ensure that allocations are properly aligned.

- Setting background target but not promote target will now leave that
  data cached on the foreground target, as it used to.

- Revert a change to when we allocate the VFS superblock, this was done
  for implementing blk_holder_ops but ended up not being needed, and
  allocating a superblock and not setting SB_BORN while we do recovery
  caused sync() calls and other things to hang.

- Assorted fixes for harmless error messages that caused concern to
  users.
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