Fix mac_veriexec version mismatch

mac_veriexec sets its version to 1, but the mac_veriexec_shaX modules which depend on it expect MAC_VERIEXEC_VERSION = 2.
Be consistent and use MAC_VERIEXEC_VERSION everywhere.
This unbreaks loading of mac_veriexec modules at boot time.

Authored by: Kornel Duleba <mindal@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D31268