tcp: account for ts offset only if tsecr not zero

We can get SYN with zero tsecr, don't apply offset in this case.

Fixes: ee684b6f ("tcp: send packets with a socket timestamp")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>