tcp: rename tcp_v4_err() skb parameter

This essentially reverts 4d1a2d9e ("Revert Backoff [v3]:
Rename skb to icmp_skb in tcp_v4_err()")

Now we have tcp_ld_RTO_revert() helper, we can use the usual
name for sk_buff parameter, so that tcp_v4_err() and
tcp_v6_err() use similar names.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>