sock_append_data() seems like a lot of wasted work.

We already pass around the fragmented SKB as a list chained by
skb_shinfo(skb)->fraglist, just pass this thing to the device and in
this way you'll avoid all of that work sock_append_data() does
entirely.

Or is there a reason you did not implement it this
way?

This is one of the uses the skb_shinfo(skb)->fraglist was intended
for.

IN FACT, this fragmentation offload you are implementing here is what
the feature bit NETIF_F_FRAGLIST was meant to indicate.