mkfs.btrfs: support fs-verity in --rootdir

[allisonkarlitskaya]

When creating a filesystem with mkfs.btrfs --rootdir, it would be nice to check if the incoming files have fs-verity enabled on them, and if so, to enable it in the created filesystem.

It's possible to use FS_IOC_READ_VERITY_METADATA to copy the data out of the running kernel, avoiding the need to redo all the calculations (although it may be useful to have this code anyway, to share with fs-verity verification in btrfs check).