Introduce xdr_stream-based XDR file handle decoders to the in-kernel
mountd client.  These are more careful than the existing decoder
functions about buffer overflows and data type and range checking.
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>