net: mac80211: use skb_list_walk_safe helper for gso segments

This is a conversion case for the new function, keeping the flow of the
existing code as intact as possible. We also switch over to using
skb_mark_not_on_list instead of a null write to skb->next.

Finally, this code appeared to have a memory leak in the case where
header building fails before the last gso segment. In that case, the
remaining segments are not freed. So this commit also adds the proper
kfree_skb_list call for the remainder of the skbs.

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