net: delete request_sock->sk_friends

The sock pointed by request_sock->friend may be freed since it does not have a
lock to protect it.
I just delete request_sock->friend since I think it is useless.

For sk_buff->friend, it has the same problem, and we use
"atomic_add(skb->truesize, &sk->sk_wmem_alloc)" to guarantee that the sock can
not be freed before the skb is freed.

Then for 3-way handshake with tcp friends enabled,
SYN->friend is NULL, SYN/ACK->friend is set in tcp_make_synack(),
and ACK->friend is set in tcp_send_ack().

Signed-off-by: Weiping Pan <wpan@redhat.com>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>