bcachefs updates for 6.9

 - Subvolume children btree; this is needed for providing a userspace
   interface for walking subvolumes, which will come later
 - Lots of improvements to directory structure checking
 - Improved journal pipelining, significantly improving performance on
   high iodepth write workloads
 - Discard path improvements: the discard path is more efficient, and no
   longer flushes the journal unnecessarily
 - Buffered write path can now avoid taking the inode lock
 - new mm helper: memalloc_flags_{save|restore}
 - mempool now does kvmalloc mempools
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