SUNRPC: Convert svcauth_unix_accept() to use xdr_stream

Done as part of hardening the server-side RPC header decoding path.

Since the server-side of the Linux kernel SunRPC implementation
ignores the contents of the Call's machinename field, there's no
need for its RPC_AUTH_UNIX authenticator to reject names that are
larger than UNX_MAXNODENAME.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>