bcachefs: bcachefs_metadata_version_disk_accounting_v3

bcachefs_metadata_version_disk_accounting_v2 erroneously had padding
bytes in disk_accounting_key, which is a problem because we have to
guarantee that all unused bytes in disk_accounting_key are zeroed.

Fortunately 6.11 isn't out yet, so it's cheap to fix this by spinning a
new version.

Reported-by: Gabriel de Perthuis <g2p.code@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>