tools/x86/kcpuid: Filter valid CPUID ranges

Next commits will introduce vendor-specific CPUID ranges like Transmeta's
0x8086000 range and Centaur's 0xc0000000.

Initially explicit vendor detection was implemented, but it turned out to
be not strictly necessary.  As Dave Hansen noted, even established tools
like cpuid(1) just tries all ranges indices, and see if the CPU responds
back with something sensible.

Do something similar at setup_cpuid_range().  Query the range's index,
and check the maximum range function value returned.  If it's within an
expected interval of [range_index, range_index + MAX_RANGE_INDEX_OFFSET],
accept the range as valid and further query its leaves.

Set MAX_RANGE_INDEX_OFFSET to a heuristic of 0xff.  That should be
sensible enough since all the ranges covered by x86-cpuid-db XML database
are:

	0x00000000	0x00000023
	0x40000000	0x40000000
	0x80000000	0x80000026
	0x80860000	0x80860007
	0xc0000000	0xc0000001

At setup_cpuid_range(), if the range's returned maximum function was not
sane, mark it as invalid by setting its number of leaves, range->nr, to
zero.

Introduce the for_each_valid_cpuid_range() iterator instead of sprinkling
"range->nr != 0" checks throughout the code.

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Ahmed S. Darwish <darwi@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20250324142042.29010-15-darwi@linutronix.de

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