[PATCH] NFSv4: Fix an Oops in the callback code.

 The changeset "trond.myklebust@fys.uio.no|ChangeSet|20050322152404|16979"
 (RPC: Ensure XDR iovec length is initialized correctly in call_header)
 causes the NFSv4 callback code to BUG() due to an incorrectly initialized
 scratch buffer.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>