To see the problem, create many hardlinks to the same file (120 should do it),
then look up paths by inode with:
ls -i
btrfs inspect inode-resolve -v $ino /mnt/btrfs
I noticed the memory layout of the fspath->val data had some irregularities
(some unnecessary gaps that stop appearing about halfway),
so I'm not sure there aren't any bugs left in it.