x86/cacheinfo: Extract out cache self-snoop checks

The logic of not doing a cache flush if the CPU declares cache self
snooping support is repeated across the x86/cacheinfo code.  Extract it
into its own function.

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>
Link: https://lore.kernel.org/r/20250324133324.23458-27-darwi@linutronix.de