bcachefs fixes for 6.8-rc6

Some more mostly boring fixes, but some not

User reported ones:
 - the BTREE_ITER_FILTER_SNAPSHOTS one fixes a really nasty performance
   bug; user reported an unter initially taking 2 seconds and then ~2
   minutes

 - kill a __GFP_NOFAIL in the buffered read path; this was a leftover
   from the trickier fix to kill __GFP_NOFAIL in readahead, where we
   can't return errors (and have to silently truncate the read
   ourselves).

   bcachefs can't use GFP_NOFAIL for folio state unlike iomap based
   filesystems because our folio state is just barely too big, 2MB
   hugepages cause us to exceed the 2 page threshhold for GFP_NOFAIL.

   additionally, the flags argument was just buggy, we weren't supplying
   GFP_KERNEL previously (!).
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