tcp: optimize some skb_shinfo(skb) uses

Compiler doesn't know skb_shinfo(skb) pointer is usually constant.

By using a temporary variable, we help generating smaller code.

For example, tcp_init_nondata_skb() is inlined after this patch.

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