tcp: fix range tcp_shifted_skb() passes to tcp_sacktag_one()

Fix the newly-SACKed range to be the range of newly-shifted bytes.

Previously - since 832d11c5 -
tcp_shifted_skb() incorrectly called tcp_sacktag_one() with the start
and end sequence numbers of the skb it passes in set to the range just
beyond the range that is newly-SACKed.

This commit also removes a special-case adjustment to lost_cnt_hint in
tcp_shifted_skb() since the pre-existing adjustment of lost_cnt_hint
in tcp_sacktag_one() now properly handles this things now that the
correct start sequence number is passed in.

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