RDS: TCP: Avoid rds connection churn from rogue SYNs

RDS: TCP: Avoid rds connection churn from rogue SYNs

Orabug: 23542064

Backport of upstream commmit c948bb5c2cc4 ("RDS: TCP: Avoid rds connection
churn from rogue SYNs")

When a rogue SYN is received after the connection arbitration
algorithm has converged, the incoming SYN should not needlessly
quiesce the transmit path, and it should not result in needless
TCP connection resets due to re-execution of the connection
arbitration logic.

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>