tcp: allow tcp_sacktag_one() to tag ranges not aligned with skbs

This commit allows callers of tcp_sacktag_one() to pass in sequence
ranges that do not align with skb boundaries, as tcp_shifted_skb()
needs to do in an upcoming fix in this patch series.

In fact, now tcp_sacktag_one() does not need to depend on an input skb
at all, which makes its semantics and dependencies more clear.

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