bcachefs fixes for 6.9-rc5

various recovery fixes:

- fixes for the btree_insert_entry being resized on path allocation
  btree_path array recently became dynamically resizable, and
  btree_insert_entry along with it; this was being observed during
  journal replay, when write buffer btree updates don't use the write
  buffer and instead use the normal btree update path
- multiple fixes for deadlock in recovery when we need to do lots of
  btree node merges; excessive merges were clocking up the whole
  pipeline
- write buffer path now correctly does btree node merges when needed
- fix failure to go RW when superblock indicates recovery passes needed
  (i.e. to complete an unfinished upgrade)

various unsafety fixes - test case contributed by a user who had two
drives out of a six drive array write out a whole bunch of garbage after
power failure

new (tiny) on disk format feature: since it appears the btree node scan
tool will be a more regular thing (crappy hardware, user error) - this
adds a 64 bit per-device bitmap of regions that have ever had btree
nodes.

a path->should_be_locked fix, from a larger patch series tightening up
invariants and assertions around btree transaction and path locking
state; this particular fix prevents us from keeping around btree_paths
that are no longer needed.
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