bcachefs fixes for v6.8-rc2

 - fix for REQ_OP_FLUSH usage; this fixes filesystems going read only
   with -EOPNOTSUPP from the block layer.

   (this really should have gone in with the block layer patch causing
   the -EOPNOTSUPP, or should have gone in before).
 - fix an allocation in non-sleepable context
 - fix one source of srcu lock latency, on devices with terrible discard
   latency
 - fix a reattach_inode() issue in fsck
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