x86/cpu: Sanitize CPUID(0x80000000) output

x86/cpu: Sanitize CPUID(0x80000000) output

CPUID(0x80000000).EAX returns the max extended CPUID leaf available.  On
x86-32 machines without an extended CPUID range, a CPUID(0x80000000)
query will just repeat the output of the last valid standard CPUID leaf
on the CPU; i.e., a garbage values.  Current tip:x86/cpu code protects against
this by doing:

	eax = cpuid_eax(0x80000000);
	c->extended_cpuid_level = eax;

	if ((eax & 0xffff0000) == 0x80000000) {
		// CPU has an extended CPUID range. Check for 0x80000001
		if (eax >= 0x80000001) {
			cpuid(0x80000001, ...);
		}
	}

This is correct so far.  Afterwards though, the same possibly broken EAX
value is used to check the availability of other extended CPUID leaves:

	if (c->extended_cpuid_level >= 0x80000007)
		...
	if (c->extended_cpuid_level >= 0x80000008)
		...
	if (c->extended_cpuid_level >= 0x8000000a)
		...
	if (c->extended_cpuid_level >= 0x8000001f)
		...

which is invalid.  Fix this by immediately setting the CPU's max extended
CPUID leaf to zero if CPUID(0x80000000).EAX doesn't indicate a valid
CPUID extended range.

While at it, add a comment, similar to kernel/head_32.S, clarifying the
CPUID(0x80000000) sanity check.

References: 8a50e5135af0 ("x86-32: Use symbolic constants, safer CPUID when enabling EFER.NX")
Fixes: 3da99c977637 ("x86: make (early)_identify_cpu more the same between 32bit and 64 bit")
Signed-off-by: Ahmed S. Darwish <darwi@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: John Ogness <john.ogness@linutronix.de>
Cc: x86-cpuid@lists.linux.dev
Link: https://lore.kernel.org/r/20250506050437.10264-3-darwi@linutronix.de