nfs: use btrfs ioctl defintions for clone

The NFS CLONE_RANGE defintion was wrong and thus never worked.  Fix this
by simply using the btrfs ioctl defintion.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>