Restore condition removed in df3747c6607b.

I think it allowed to avoid some TX thread wakeups while the socket
buffer is full. But add there another options if ic_check_send_space
is set, which means socket just reported that new space appeared, so
it may have sense to pull more data from ic_to_send for better TX
coalescing.

MFC after: 1 week

(cherry picked from commit aff9b9ee894e3e6b6d8c7e4182d6b973804df853)