net: tcp: Avoid partial ACK canceling retransmission timer

At any ack, the retransmission timer was cancelled. This means when an ACK
is only partially acknowledging pending data, followed by a packet loss,
the connection ended in a deadlock eventually timing out.
By checking if there is any pending data for transmission before canceling
the retransmission timer, there is no risk of this lock-up any more.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>