SUNRPC: Fix bytes-per-op accounting for RPC over UDP

NFS performance metrics reported zero bytes sent per op when mounting with
UDP.  The UDP socket transport wasn't properly counting the number of bytes
sent.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>