RDS: TCP: Force every connection to be initiated by numerically smaller IP address

When 2 RDS peers initiate an RDS-TCP connection simultaneously,
there is a potential for "duelling syns" on either/both sides.
See commit 241b2719 ("RDS-TCP: Reset tcp callbacks if re-using an
outgoing socket in rds_tcp_accept_one()") for a description of this
condition, and the arbitration logic which ensures that the
numerically large IP address in the TCP connection is bound to the
RDS_TCP_PORT ("canonical ordering").

The rds_connection should not be marked as RDS_CONN_UP until the
arbitration logic has converged for the following reason. The sender
may start transmitting RDS datagrams as soon as RDS_CONN_UP is set,
and since the sender removes all datagrams from the rds_connection's
cp_retrans queue based on TCP acks. If the TCP ack was sent from
a tcp socket that got reset as part of duel aribitration (but
before data was delivered to the receivers RDS socket layer),
the sender may end up prematurely freeing the datagram, and
the datagram is no longer reliably deliverable.

This patch remedies that condition by making sure that, upon
receipt of 3WH completion state change notification of TCP_ESTABLISHED
in rds_tcp_state_change, we mark the rds_connection as RDS_CONN_UP
if, and only if, the IP addresses and ports for the connection are
canonically ordered. In all other cases, rds_tcp_state_change will
force an rds_conn_path_drop(), and rds_queue_reconnect() on
both peers will restart the connection to ensure canonical ordering.

A side-effect of enforcing this condition in rds_tcp_state_change()
is that rds_tcp_accept_one_path() can now be refactored for simplicity.
It is also no longer possible to encounter an RDS_CONN_UP connection in
the arbitration logic in rds_tcp_accept_one().

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>