net: tcp: Handle retransmitted packets from peer.

When we receive a packet with the sequence we already seen (and
processed), the most likely cause of it is that our ACK was lost,
and peer has to retransmit that packet. Then, we should just ACK
it, because otherwise peer will retransmit it again and again,
falling into exponential backoff and hosing the entire TCP
connection.

This makes changes to send_ack(), adding a flag to force sending
an ACK regardless of its cached status, and remove inline modifier,
as the function is big and called from many places.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>