commit db44bac4 upstream.

Use a couple shortcuts that will simplify a following bugfix.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
[bwh: Backported to 3.2: in nfs3svc_decode_writeargs(), dlen doesn't include
 tail]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>