net: avoid unnecessary sock_flag() check when enable timestamp

The sock_flag() check is alreay inside sock_enable_timestamp(), so it is
unnecessary checking it in the caller.

    void sock_enable_timestamp(struct sock *sk, int flag)
    {
        if (!sock_flag(sk, flag)) {
            ...
        }
    }

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>