tcp: fix wrong RTO timeout when received SACK reneging

This commit fix wrong RTO timeout when received SACK reneging.

When an ACK arrived pointing to a SACK reneging, tcp_check_sack_reneging()
will rearm the RTO timer for min(1/2*srtt, 10ms) into to the future.

But since the commit 62d9f1a6 ("tcp: fix TLP timer not set when
CA_STATE changes from DISORDER to OPEN") merged, the tcp_set_xmit_timer()
is moved after tcp_fastretrans_alert()(which do the SACK reneging check),
so the RTO timeout will be overwrited by tcp_set_xmit_timer() with
icsk_rto instead of 1/2*srtt.

Here is a packetdrill script to check this bug:
0     socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0    bind(3, ..., ...) = 0
+0    listen(3, 1) = 0

// simulate srtt to 100ms
+0    < S 0:0(0) win 32792 <mss 1000, sackOK,nop,nop,nop,wscale 7>
+0    > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 7>
+.1    < . 1:1(0) ack 1 win 1024

+0    accept(3, ..., ...) = 4

+0    write(4, ..., 10000) = 10000
+0    > P. 1:10001(10000) ack 1

// inject sack
+.1    < . 1:1(0) ack 1 win 257 <sack 1001:10001,nop,nop>
+0    > . 1:1001(1000) ack 1

// inject sack reneging
+.1    < . 1:1(0) ack 1001 win 257 <sack 9001:10001,nop,nop>

// we expect rto fired in 1/2*srtt (50ms)
+.05    > . 1001:2001(1000) ack 1

This fix remove the FLAG_SET_XMIT_TIMER from ack_flag when
tcp_check_sack_reneging() set RTO timer with 1/2*srtt to avoid
being overwrited later.

Fixes: 62d9f1a6

 ("tcp: fix TLP timer not set when CA_STATE changes from DISORDER to OPEN")
Signed-off-by: Fred Chen <fred.chenchen03@gmail.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Tested-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>