tcp: change data receiver flowlabel after one dup

This commit changes the data receiver repath behavior to occur after
receiving a single duplicate. This can help recover ACK connectivity
quicker if a TLP was sent along a nonworking path.

For instance, consider the case where we have an initially nonworking
forward path and reverse path and subsequently switch to only working
forward paths. Before this patch we would have the following behavior.

+---------+--------+--------+----------+----------+----------+
| Event   | For FL | Rev FL | FP Works | RP Works | Data Del |
+---------+--------+--------+----------+----------+----------+
| Initial | A      | 1      | N        | N        | 0        |
+---------+--------+--------+----------+----------+----------+
| TLP     | A      | 1      | N        | N        | 0        |
+---------+--------+--------+----------+----------+----------+
| RTO 1   | B      | 1      | Y        | N        | 1        |
+---------+--------+--------+----------+----------+----------+
| RTO 2   | C      | 1      | Y        | N        | 2        |
+---------+--------+--------+----------+----------+----------+
| RTO 3   | D      | 2      | Y        | Y        | 3        |
+---------+--------+--------+----------+----------+----------+

This patch gets rid of at least RTO 3, avoiding additional unnecessary
repaths of a working forward path to a (potentially) nonworking one.

In addition, this commit changes the behavior to avoid repathing upon
rx of duplicate data if the local endpoint is in CA_Loss (in which
case the RTOs will already be changing the outgoing flowlabel).

Signed-off-by: David Morley <morleyd@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Tested-by: David Morley <morleyd@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>