tcp: increase tcps_sc_recvcookie only in the syncache_expand()

The syncookie_expand() is called from syncookie_cmp() in INVARIANTS mode
to confirm that values calculated via syncookies mechanism match those
stored in the syncache entry. This creates a counting bug, that with
INVARIANTS every successful use of syncache also counts as use of a
syncookie.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D50897