sctp: allow increasing cwnd regardless of ctsn moving or not

As per RFC4960 Errata 3.22, this condition is not needed anymore as it
could cause the partial_bytes_acked to not consider the TSNs acked in
the Gap Ack Blocks although they were received by the peer successfully.

This patch thus drops the check for new Cumulative TSN Ack Point,
leaving just the flight_size < cwnd one.

See-also: https://tools.ietf.org/html/draft-ietf-tsvwg-rfc4960-errata-01#section-3.22


Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>