NFS: Extract the xdr_init_encode/decode() calls from decode_compound

Clean up: Move the xdr_init_encode() and xdr_init_decode() calls
into the dispatcher, just like the NFSD and lockd dispatchers.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>