tcp: clear tp->packets_out when purging write queue

tcp: clear tp->packets_out when purging write queue

Clear tp->packets_out when purging the write queue, otherwise
tcp_rearm_rto() mistakenly assumes TCP write queue is not empty.
This results in NULL pointer dereference.

Also, remove the redundant `tp->packets_out = 0` from
tcp_disconnect(), since tcp_disconnect() calls
tcp_write_queue_purge().

Fixes: a27fd7a8ed38 (tcp: purge write queue upon RST)
Reported-by: Subash Abhinov Kasiviswanathan <[email protected]>
Reported-by: Sami Farin <[email protected]>
Tested-by: Sami Farin <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Signed-off-by: Soheil Hassas Yeganeh <[email protected]>
Acked-by: Yuchung Cheng <[email protected]>
Acked-by: Neal Cardwell <[email protected]>
Signed-off-by: David S. Miller <[email protected]>