tcp: [BUG] call dst_release() in tcp_v4_conn_request()

Before merging TFO patch set, in tcp_v4_conn_request() we always call
__tcp_v4_send_synack() to send a packet with syn and ack flag.  In
__tcp_v4_send_synack() function it always calls dst_release() to
decrease the ref count for a dst_entry.  But after applying TFO patch
set this function is never called.  So we will meet a bug like below:

unregister_netdevice: waiting for lo to become free.  Usage count = 3

Patch-mainline: in-house
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>