bcachefs fixes for v6.9-rc4

Notable user impacting bugs

- On multi device filesystems, recovery was looping in
  btree_trans_too_many_iters(). This checks if a transaction has touched
  too many btree paths (because of iteration over many keys), and isuses
  a restart to drop unneeded paths. But it's now possible for some paths
  to exceed the previous limit without iteration in the interior btree
  update path, since the transaction commit will do alloc updates for
  every old and new btree node, and during journal replay we don't use
  the btree write buffer for locking reasons and thus those updates use
  btree paths when they wouldn't normally.

- Fix a corner case in rebalance when moving extents on a durability=0
  device. This wouldn't be hit when a device was formatted with
  durability=0 since in that case we'll only use it as a write through
  cache (only cached extents will live on it), but durability can now be
  changed on an existing device.

- bch2_get_acl() could rarely forget to handle a transaction restart;
  this manifested as the occasional missing acl that came back after
  dropping caches.

- Fix a major performance regression on high iops multithreaded write
  workloads (only since 6.9-rc1); a previous fix for a deadlock in the
  interior btree update path to check the journal watermark introduced a
  dependency on the state of btree write buffer flushing that we didn't
  want.

- Assorted other repair paths and recovery fixes.
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