btrfs_get_verity_descriptor
ret = btrfs_get_verity_descriptor(&inode->vfs_inode, NULL, 0);
ret = btrfs_get_verity_descriptor(&inode->vfs_inode, sctx->verity_descriptor, ret);
.get_verity_descriptor = btrfs_get_verity_descriptor,
int btrfs_get_verity_descriptor(struct inode *inode, void *buf, size_t buf_size);