bcachefs fixes for 6.7 stable

"bcachefs: fix simulateously upgrading & downgrading" is the important
one here. This fixes a really nasty bug where in a rare situation we
wouldn't downgrade; we'd write a superblock where the version number is
higher than the currently supported version.

This caused total failure to mount multi device filesystems with the
splitbrain checking in 6.8, since now we wouldn't be updating the member
sequence numbers used for splitbrain checking, but the version number
said we would be - and newer versions would attempt to kick every device
out of the fs.
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