tcp: deferring in middle of queue makes very little sense

If skb can be sent right away, we certainly should do that
if it's in the middle of the queue because it won't get
more data into it.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>