tcp: Don't allow persist timer if TCP connection is not established yet

tcp: Don't allow persist timer if TCP connection is not established yet

This probably could move the un-updated snd_nxt panic earlier.

The dump of the panic in http://bugs.dragonflybsd.org/issue1939 shows
- snd_nxt is less than snd_una
- A persist timer was fired (frame 16, tp->tt_msg->tt_prev_tasks).
- The TCP segment triggered the panic has SYN|ACK (frame 17, th->th_flags).
  This TCP segment is considered as valid (frame 17, list), so tp->t_state
  was SYN_SENT.

This explains why snd_nxt is less than snd_una:
If tcp_output() is called by persist timer, then the persist timer is
active and the "forced" is turned on, this causes the snd_nxt not updated
at all.

MISSING CHIAN IN THE LINK:
How is the persist timer got set in the SYN_SENT in the first place?

Hope the new panic could lift the veil...