net: minor: tcp: use tcp_skb_mss helper in tcp_tso_segment

We have the minimal inline helper tcp_skb_mss to access
skb_shinfo(skb)->gso_size, so also use it here to get mss.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>