tipc: failed transmissions should return error

When a message could not be sent out because the destination node
or link could not be found, the full message size is returned from
sendmsg() as if it had been sent successfully. An application will
then get a false indication that it's making forward progress. This
problem has existed since the initial commit in 2.6.16.

We change this to return -ENETUNREACH if the message cannot be
delivered due to the destination node/link being unavailable. We
also get rid of the redundant tipc_reject_msg call since freeing
the buffer and doing a tipc_port_iovec_reject accomplishes exactly
the same thing.

Signed-off-by: Erik Hugne <erik.hugne@ericsson.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>