ipv4: tcp: set unicast_sock uc_ttl to -1

Set unicast_sock uc_ttl to -1 so that we select the right ttl,
instead of sending packets with a 0 ttl.

Bug added in commit be9f4a44

 (ipv4: tcp: remove per net tcp_sock)

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