tcp: fix fack_count accounting on tcp_shift_skb_data()

tcp: fix fack_count accounting on tcp_shift_skb_data()

v4.15 or since commit 737ff314563 ("tcp: use sequence distance to
detect reordering") had switched from the packet-based FACK tracking
to sequence-based.

v4.14 and older still have the old logic and hence on
tcp_skb_shift_data() needs to retain its original logic and have
@fack_count in sync. In other words, we keep the increment of pcount with
tcp_skb_pcount(skb) to later used that to update fack_count. To make it
more explicit we track the new skb that gets incremented to pcount in
@next_pcount, and we get to avoid the constant invocation of
tcp_skb_pcount(skb) all together.

Orabug: 29890820
Fixes: 1b56e4cb5dec ("tcp: limit payload size of sacked skbs")
Reported-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Reviewed-by: John Haxby <john.haxby@oracle.com>
Reviewed-by: Rao Shoaib <rao.shoaib@oracle.com>>>>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>