bcachefs fixes for 6.11-rc4

- New on disk format version, bcachefs_metadata_version_disk_accounting_inum

This adds one more disk accounting counter, which counts disk usage and
number of extents per inode number. This lets us track fragmentation,
for implementing defragmentation later, and it also counts disk usage
per inode in all snapshots, which will be a useful thing to expose to
users.

- One performance issue we've observed is threads spinning when they
should be waiting for dirty keys in the key cache to be flushed by
journal reclaim, so we now have hysteresis for the waiting thread, as
well as improving the tracepoint and a new time_stat, for tracking time
blocked waiting on key cache flushing.

And, various assorted smaller fixes.
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