sysfs/cpu: Add vulnerability folder

sysfs/cpu: Add vulnerability folder

As the meltdown/spectre problem affects several CPU architectures, it makes
sense to have common way to express whether a system is affected by a
particular vulnerability or not. If affected the way to express the
mitigation should be common as well.

Create /sys/devices/system/cpu/vulnerabilities folder and files for
meltdown, spectre_v1 and spectre_v2.

Allow architectures to override the show function.

Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Reviewed-by: Konrad Rzeszutek Wilk <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Woodhouse <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]