NFS: Prevent garbage cinfo->ds from leaking out

This is a bugfix that applies on top of the previous directio patches,
that fixes a bug introduced in "NFS: create struct nfs_commit_info".

Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>