It's interesting, however, that the number of TCP packets is exactly twice
that of the number of non-linear sk_buffs we saw in tbnet_start_xmit. Not that
it's suspicious, if anything (and because we see those TCP packets in blue) it
tells us that the handling of non-linear skbs is not the problem in
tbnet_start_xmit.
But why twice? Or is this a red herring?

I've confirmed that all UDP skb's we transmit are linear.
All non-linear skb's are TCP.