tipc: improve error validations for sockets in CONNECTING state

Until now, the checks for sockets in CONNECTING state was based on
the assumption that the incoming message was always from the
peer's accepted data socket.

However an application using a non-blocking socket sends an implicit
connect, this socket which is in CONNECTING state can receive error
messages from the peer's listening socket. As we discard these
messages, the application socket hangs as there due to inactivity.
In addition to this, there are other places where we process errors
but do not notify the user.

In this commit, we process such incoming error messages and notify
our users about them using sk_state_change().

Signed-off-by: Parthasarathy Bhuvaragan <parthasarathy.bhuvaragan@ericsson.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>