net: udp: use skb_list_walk_safe helper for gso segments

This is a straight-forward conversion case for the new function,
iterating over the return value from udp_rcv_segment, which actually is
a wrapper around skb_gso_segment.

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