sctp: Correctly start rtx timer on new packet transmissions.

sctp: Correctly start rtx timer on new packet transmissions.

Commit 0cd56f342cdd7251ccc58d79ed8cf59553575d48
(sctp: Start T3-RTX timer when fast retransmitting lowest TSN)
introduced a regression where it was possible to forcibly
restart the sctp retransmit timer at the transmission of any
new chunk.  This resulted in much longer timeout times and
sometimes hung sctp connections.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>