nfsd4: minor NFSv2/v3 write decoding cleanup

commit db44bac41bbfc0c0d9dd943092d8bded3c9db19b 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>

diff --cc fs/nfsd/nfs3xdr.c
Simple merge

diff --cc fs/nfsd/nfsxdr.c
Simple merge