lib/commit: Directly use FICLONE for payload link
The idea of payload linking is to reflink between objects where
possible. Instead of relying on `glnx_regfile_copy_bytes` to hit the
`FICLONE` path, just call `FICLONE` directly. At that point in the code,
we've already established that the source and dest repos are on the same
filesystem and that it supports `FICLONE`.
Related: https://gitlab.gnome.org/GNOME/libglnx/-/merge_requests/41
Related: https://github.com/ostreedev/ostree/pull/2684#issuecomment-
1204068437