nfs: return compound hdr.status when there are no op replies

When there are no op replies encoded in the compound reply
hdr.status still contains the overall status of the compound
rpc.  This can happen, e.g., when the server returns a
NFS4ERR_MINOR_VERS_MISMATCH error.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>