SUNRPC: Remove the rpc_stat variable in svc_process_common()

There's no RPC header field called rpc_stat; more precisely, the
variable appears to be recording an accept_stat value. But it looks
like we don't need to preserve this value at all, actually, so
simply remove the variable.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>