x86/local_apic.c: Properly calculate the number of LVT entries

First, the CMCI entry index is APIC_LVT_MAX, so it was excluded
unconditionall [1].

Second, the number of entries is reported by the version register, and
we must not access past the last reported entry.

Reported by: olivier [1]
Fixes: 11f954b021a1aadde1d03d40ed5d6b529e14da98
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D54773