NFSv4: Kill braindead gcc warnings

nfs4_open_revalidate: 'res' may be used uninitialized
nfs4_callback_compound: ‘hdr_res.nops’ may be used uninitialized
			'op_nr’ may be used uninitialized
encode_getattr_res: ‘savep’ may be used uninitialized

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