bcachefs updates for 6.8:

 - btree write buffer rewrite: instead of adding keys to the btree write
   buffer at transaction commit time, we know journal them with a
   different journal entry type and copy them from the journal to the
   write buffer just prior to journal write.

   This reduces the number of atomic operations on shared cachelines
   in the transaction commit path and is a signicant performance
   improvement on some workloads: multithreaded 4k random writes went
   from ~650k iops to ~850k iops.

 - Bring back optimistic spinning for six locks: the new implementation
   doesn't use osq locks; instead we add to the lock waitlist as normal,
   and then spin on the lock_acquired bit in the waitlist entry, _not_
   the lock itself.

 - BCH_IOCTL_DEV_USAGE_V2, which allows for new data types
 - BCH_IOCTL_OFFLINE_FSCK, which runs the kernel implementation of fsck
   but without mounting: useful for transparently using the kernel
   version of fsck from 'bcachefs fsck' when the kernel version is a
   better match for the on disk filesystem.

 - BCH_IOCTL_ONLINE_FSCK: online fsck. Not all passes are supported yet,
   but the passes that are supported are fully featured - errors may be
   corrected as normal.

   The new ioctls use the new 'thread_with_file' abstraction for kicking
   off a kthread that's tied to a file descriptor returned to userspace
   via the ioctl.

 - btree_paths within a btree_trans are now dynamically growable,
   instead of being limited to 64. This is important for the
   check_directory_structure phase of fsck, and also fixes some issues
   we were having with btree path overflow in the reflink btree.

 - Trigger refactoring; prep work for the upcoming disk space accounting
   rewrite

 - Numerous bugfixes :)
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