cp: truncate destination when --reflink is set

This is needed in order to allow overriding an existing file.

```
$ 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 --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']
$
$ # With truncate(true)
$ cargo run cp --reflink=always /tmp/disk/src1 /tmp/disk/dst3
    Finished dev [unoptimized + debuginfo] target(s) in 7.98s
     Running `target/debug/coreutils cp --reflink=always /tmp/disk/src1 /tmp/disk/dst3`
$ cargo run cp --reflink=always /tmp/disk/src2 /tmp/disk/dst3
    Finished dev [unoptimized + debuginfo] target(s) in 0.27s
     Running `target/debug/coreutils cp --reflink=always /tmp/disk/src2 /tmp/disk/dst3`
$ cat /tmp/disk/dst3
success
```

src/uu/cp/src/cp.rs

@@ -1535,7 +1535,7 @@ fn copy_on_write_linux(
     let src_file = File::open(source).context(context)?;
     let dst_file = OpenOptions::new()
         .write(true)
-        .truncate(false)
+        .truncate(true)
         .create(true)
         .open(dest)
         .context(context)?;

tests/by-util/test_cp.rs

@@ -1388,6 +1388,50 @@ fn test_closes_file_descriptors() {
         .succeeds();
 }
 
+#[cfg(any(target_os = "linux", target_os = "android"))]
+#[test]
+fn test_reflink_always() {
+    let scene = TestScenario::new(util_name!());
+
+    const DISK: &str = "disk.img";
+    const MOUNTPOINT: &str = "mount/";
+    let src1_path: &str = &vec![MOUNTPOINT, "src1"].concat();
+    let src2_path: &str = &vec![MOUNTPOINT, "src2"].concat();
+    let dst_path: &str = &vec![MOUNTPOINT, "dst"].concat();
+
+    scene.ccmd("truncate")
+        .args(&["-s", "128M", DISK])
+        .succeeds();
+
+    scene.cmd("mkfs.btrfs")
+        .args(&[DISK])
+        .succeeds();
+
+    scene.fixtures.mkdir(MOUNTPOINT);
+
+    scene.cmd("mount")
+        .args(&[DISK, MOUNTPOINT])
+        .succeeds();
+
+    scene.fixtures.make_file(src1_path);
+    scene.fixtures.write_bytes(src1_path, &[0x64; 8192]);
+
+    scene.fixtures.make_file(src2_path);
+    scene.fixtures.write(src2_path, "other data");
+
+    scene.ucmd()
+        .args(&["--reflink=always", src1_path, dst_path])
+        .succeeds();
+
+    scene.ucmd()
+        .args(&["--reflink=always", src2_path, dst_path])
+        .succeeds();
+
+    scene.cmd("umount")
+        .args(&[MOUNTPOINT])
+        .succeeds();
+}
+
 #[test]
 fn test_copy_dir_symlink() {
     let (at, mut ucmd) = at_and_ucmd!();