netfilter: conntrack: correct window scaling with retransmitted SYN

commit c7aab4f1 ("netfilter: nf_conntrack_tcp: re-init for syn packets
only") introduces a bug where SYNs in ORIGINAL direction on reused 5-tuple
result in incorrect window scale negotiation. This commit merged the SYN
re-initialization and simultaneous open or SYN retransmits cases. Merging
this block added the logic in tcp_init_sender() that performed window scale
negotiation to the retransmitted syn case. Previously. this would only
result in updating the sender's scale and flags. After the merge the
additional logic results in improperly clearing the scale in ORIGINAL
direction before any packets in the REPLY direction are received. This
results in packets incorrectly being marked invalid for being
out-of-window.

This can be reproduced with the following trace:

Packet Sequence:
> Flags [S], seq 1687765604, win 62727, options [.. wscale 7], length 0
> Flags [S], seq 1944817196, win 62727, options [.. wscale 7], length 0

In order to fix the issue, only evaluate window negotiation for packets
in the REPLY direction. This was tested with simultaneous open, fast
open, and the above reproduction.

Fixes: c7aab4f1

 ("netfilter: nf_conntrack_tcp: re-init for syn packets only")
Signed-off-by: Ryan Schaefer <ryanschf@amazon.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>