More bcachefs bugfixes for 6.7, and forwards compatibility work:

 - fix for a nasty extents + snapshot interaction, reported when reflink
   of a snapshotted file wouldn't complete but turned out to be a more
   general bug
 - fix for an invalid free in dio write path when iov vector was longer
   than our inline vecotr
 - fix for a buffer overflow in the nocow write path - BCH_REPLICAS_MAX
   doesn't actually limit the number of pointers in an extent when
   cached pointers are included
 - RO snapshots are actually RO now
 - And, a new superblock section to avoid future breakage when the disk
   space acounting rewrite rolls out: the new superblock section
   describes versions that need work to downgrade, where the work
   required is a list of recovery passes and errors to silently fix.
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