lockd: Introduce new-style XDR functions for NLMv4

We'd like to prevent local buffer overflows caused by malicious or
broken servers.  New xdr_stream style decoders can do that.

For efficiency, we also want to be able to pass xdr_streams from
call_encode() to all XDR encoding functions, rather than building
an xdr_stream in every XDR encoding function in the kernel.

Same idea as the NLM v3 XDR overhaul.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>