net: myri10ge: use skb_list_walk_safe helper for gso segments

This is a straight-forward conversion case for the new function, and
while we're at it, we can remove a null write to skb->next by replacing
it with skb_mark_not_on_list.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>