[TCP]: Check num sacks in SACK fast path

We clear the unused parts of the SACK cache, This prevents us from mistakenly
taking the cache data if the old data in the SACK cache is the same as the data
in the SACK block. This assumes that we never receive an empty SACK block with
start and end both at zero.

Signed-off-by: Baruch Even <baruch@ev-en.org>
Signed-off-by: David S. Miller <davem@davemloft.net>