SUNRPC: rpc_decode_header() must always return a non-zero value on error

Ensure that when the "garbage args" case falls through, we do set
an error of EIO.

Fixes: a0584ee9

 ("SUNRPC: Use struct xdr_stream when decoding...")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>