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

[kib]

Summary

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


x86/local_apic.c: add lapic_maxlvt() helper

that calculates the max index of the present LVT entry from the value of
the LAPIC version register.

[kib]

Add helper

[markj]

sys/x86/x86/local_apic.c

322

You can use this helper in a few other places, e.g., lapic_dump().

[kib]

Use lapic_maxlvt() everywhere.