[TCP]: Fix fastpath_cnt_hint when GSO skb is partially ACKed

When only GSO skb was partially ACKed, no hints are reset,
therefore fastpath_cnt_hint must be tweaked too or else it can
corrupt fackets_out. The corruption to occur, one must have
non-trivial ACK/SACK sequence, so this bug is not very often
that harmful. There's a fackets_out state reset in TCP because
fackets_out is known to be inaccurate and that fixes the issue
eventually anyway.

In case there was also at least one skb that got fully ACKed,
the fastpath_skb_hint is set to NULL which causes a recount for
fastpath_cnt_hint (the old value won't be accessed anymore),
thus it can safely be decremented without additional checking.

Reported by Cedric Le Goater <clg@fr.ibm.com>
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -2420,6 +2420,9 @@ static int tcp_tso_acked(struct sock *sk, struct sk_buff *skb,
 			__u32 dval = min(tp->fackets_out, packets_acked);
 			tp->fackets_out -= dval;
 		}
+		/* hint's skb might be NULL but we don't need to care */
+		tp->fastpath_cnt_hint -= min_t(u32, packets_acked,
+					       tp->fastpath_cnt_hint);
 		tp->packets_out -= packets_acked;
 
 		BUG_ON(tcp_skb_pcount(skb) == 0);