Bug: trying to create reflink on different device results in empty file

[Koen De Wit <koen.de.wit@oracle.com>]

When trying to create a clone (reflink) of a file on a different device, 
you'll get this error:
     Invalid cross-device link

However, an empty file is created on the target location. An invalid 
clone operation should not result in the creation of a file.


Example:

# mount
(...)
/dev/sdb1 on /mnt/diskb type btrfs (rw)
/dev/sdf1 on /mnt/diskf type btrfs (rw)

# ls -l diskb
total 4
-rw-r--r-- 1 root root 11 Dec 11 22:45 testfile.txt

# ls -l diskf
total 0

# cp --reflink diskb/testfile.txt diskf/testcopy.txt
cp: failed to clone `diskf/testcopy.txt' from `diskb/testfile.txt': 
Invalid cross-device link

# ls -l diskf
total 0
-rw-r--r-- 1 root root 0 Dec 11 22:56 testcopy.txt