Do not return number of bytes written for ioctl CIFS_IOC_COPYCHUNK_FILE

commit 620d8745 ("Introduce cifs_copy_file_range()") changes the
behaviour of the cifs ioctl call CIFS_IOC_COPYCHUNK_FILE. In case of
successful writes, it now returns the number of bytes written. This
return value is treated as an error by the xfstest cifs/001. Depending
on the errno set at that time, this may or may not result in the test
failing.

The patch fixes this by setting the return value to 0 in case of
successful writes.

Fixes: commit 620d8745 ("Introduce cifs_copy_file_range()")
Reported-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Acked-by: Pavel Shilovsky <pshilov@microsoft.com>
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <smfrench@gmail.com>