tcp: fix tcp_shifted_skb() adjustment of lost_cnt_hint for FACK

This commit ensures that lost_cnt_hint is correctly updated in
tcp_shifted_skb() for FACK TCP senders. The lost_cnt_hint adjustment
in tcp_sacktag_one() only applies to non-FACK senders, so FACK senders
need their own adjustment.

This applies the spirit of 1e5289e1 -
except now that the sequence range passed into tcp_sacktag_one() is
correct we need only have a special case adjustment for FACK.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>