NFS: Use unsigned intermediates for manipulating header lengths (NFSv3 XDR)

Clean up: prevent length underflow and mixed sign comparisons when
unmarshalling NFS version 3 read, readdir, and readlink replies.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>