Fix for interaction problems between TCP SACK and TCP Signature.
If TCP Signatures are enabled, the maximum allowed sack blocks aren't
going to fit. The fix is to compute how many sack blocks fit and tack
these on last. Also on SYNs, defer padding until after the SACK
PERMITTED option has been added.

Found by: Mohan Srinivasan.
Submitted by: Mohan Srinivasan, Noritoshi Demizu.
Reviewed by: Raja Mukerji.