bcachefs fixes for 6.11-rc5

Lots of little fixes and two big items, which were orgiinally slated for
the 6.12 merge window but turned out to be pretty important.

The little stuff includes assorted syzbot fixes and some upgrade fixes
for old (pre 1.0) filesystems, and a fix for moving data off a device
that was switched to durability=0 after data had been written to it.

The big items are:
- rhashtable conversion for VFS inodes cache
  Thas was slated for the 6.12 merge window, but a deadlock was
  uncovered in __wait_for_freeing_inode(); bcachefs inverts the usual
  locking between the VFS inode cache and on disk (btree) locking, with
  some advantages and some extra trickyness. The result was that we were
  waiting (via __wait_for_freeing_inode()) on the evict -> clear_inode()
  path with btree locks held, which was a rare deadlock but undoubtedly
  also one of the sources of the SRCU warnings.

- new data structure for managing freelists in btree key cache
  This eliminates the btree key cache lock, and associated lock
  contention. User feedback is that this resolves the main source of the
  SRCU warnings we've been seeing - which means that on some of the big
  multithreaded workloads people are running the lock contention was
  really bad (threads piling up and causing O(n^2) wait times), if it
  was able to trigger a 10 second delay warning.

On the test reported by
https://lore.kernel.org/linux-bcachefs/CAGudoHGenxzk0ZqPXXi1_QDbfqQhGHu+wUwzyS6WmfkUZ1HiXA@mail.gmail.com/

We're now 4x faster than xfs on creatrees, roughly even on walktrees,
with consistent run to run performance; dominant factor in profiles is
lru lock contention.
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