[TCP]: Extract tcp_match_queue_to_sack from sacktag code

This is necessary for upcoming DSACK bugfix. Reduces sacktag
length which is not very sad thing at all... :-)

Notice that there's a need to handle out-of-mem at caller's
place.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>