rds: tcp: compute m_ack_seq as offset from ->write_seq

rds: tcp: compute m_ack_seq as offset from ->write_seq

rds-tcp uses m_ack_seq to track the tcp ack# that indicates
that the peer has received a rds_message. The m_ack_seq is
used in rds_tcp_is_acked() to figure out when it is safe to
drop the rds_message from the RDS retransmit queue.

The m_ack_seq must be calculated as an offset from the right
edge of the in-flight tcp buffer, i.e., it should be based on
the ->write_seq, not the ->snd_nxt.

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit b589513e6354a5fd6934823b7fd66bffad41137a)

Orabug: 28085214

Signed-off-by: Hans Westgaard Ry <hans.westgaard.ry@oracle.com>
Reviewed-by: Qing Huang <qing.huang@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>