tcp: tcp_v4_err() icmp skb is named icmp_skb

tcp: tcp_v4_err() icmp skb is named icmp_skb

I missed the fact that tcp_v4_err() differs from tcp_v6_err().

After commit e182eb83c960 ("Rename skb to icmp_skb in tcp_v4_err()")
the skb argument has been renamed to icmp_skb only in one function.

I will in a future patch reconciliate these functions to avoid
this kind of confusion.

Fixes: 1e209d95d612 ("tcp: allow traceroute -Mtcp for unpriv users")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>