NFS: Update xdr_encode_foo() functions that we're keeping

Clean up.

The new helper functions are kept in order by section of RFC 1094.
Move the two timestamp encoders we're keeping, update their coding
style, and refresh their documenting comments.

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>