BUG/MINOR: backend: inspect request not response buffer to check for TFO

BUG/MINOR: backend: inspect request not response buffer to check for TFO

In 2.6, do_connect_server() was introduced by commit 0a4dcb65f ("MINOR:
stream-int/backend: Move si_connect() in the backend scope") and changed
the approach to work with a stream instead of a stream-interface. However
si_oc(si) was wrongly turned to &s->res instead of &s->req, which breaks
TFO by always inspecting the response channel to figure whether there are
data pending.

This fix can be backported to all versions till 2.6.