tcp: use SB_AUTOSIZE flag to tell if socket buffer was set

[glebius]

Summary

The check against V_tcp_sendspace is not a correct one, as a buffer may
grow larger than the initial value. The conjunction was always false up
until 587c6c121504, and only after it the bug surfaced.

If we already grow our buffer past the value stored in the hostcache,
prefer our value.