tcp: micro-optimize SYN-cookie expansion

Only compute wscale when it is actually used. While there, change the
type of wscale to u_int as suggested by glebius.
No functional change intended.

Reviewed by: glebius, rscheff (older version)
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52296

(cherry picked from commit 341d1aabc13e47911d2eb38e857b90f7d356134e)