tcp: micro-optimize SYN-cookie expansion

[tuexen]

Summary

Only compute wscale when it is actually used.
No functional change intended.

[glebius]

sys/netinet/tcp_syncache.c

2327

I'd suggest to make it u_int. Negative right operand of << is undefined behavior. I know we don't got into negative here, but using unsigned operand would make it tidier.

[tuexen]

Use u_int instead of int as the type for wscale as suggested by glebius@.