cp is unable to override files with --reflink for linux

[pimzero]

The issue seems to be related to the use of truncate(false) for the destination of the copy when --reflink is present. My understanding of Rust documentation is that we want to truncate the file.

Below is an example comparing GNU cp to uutils cp:

$ truncate -s 512M /tmp/disk.img
$ mkfs.btrfs /tmp/disk.img
[...]
$ mkdir /tmp/disk
$ sudo mount /tmp/disk.img /tmp/disk
$ sudo chown $(id -u):$(id -g) -R /tmp/disk
$ for i in $(seq 0 8192); do echo -ne 'a' >>/tmp/disk/src1; done
$ echo "success" >/tmp/disk/src2
$
$ # GNU ls supports overriding files created with `--reflink`
$ cp --version
cp (GNU coreutils) 9.1
[...]
$ cp --reflink=always /tmp/disk/src1 /tmp/disk/dst1
$ cp --reflink=always /tmp/disk/src2 /tmp/disk/dst1
$ cat /tmp/disk/dst1
success
$
$ # Now testing with uutils
$ cargo run cp --reflink=always /tmp/disk/src1 /tmp/disk/dst2
    Finished dev [unoptimized + debuginfo] target(s) in 0.25s
     Running `target/debug/coreutils cp --reflink=always /tmp/disk/src1 /tmp/disk/dst2`
$ cargo run cp --reflink=always /tmp/disk/src2 /tmp/disk/dst2
    Finished dev [unoptimized + debuginfo] target(s) in 0.26s
     Running `target/debug/coreutils cp --reflink=always /tmp/disk/src2 /tmp/disk/dst2`
cp: failed to clone "/tmp/disk/src2" from "/tmp/disk/dst2": Invalid argument (os error 22)
$ cat /tmp/disk/dst2
[lots of 'a']