KVM: x86: Use cpu_feature_enabled() for PKU instead of #ifdef

KVM: x86: Use cpu_feature_enabled() for PKU instead of #ifdef

Replace an #ifdef on CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS with a
cpu_feature_enabled() check on X86_FEATURE_PKU.  The macro magic of
DISABLED_MASK_BIT_SET() means that cpu_feature_enabled() provides the
same end result (no code generated) when PKU is disabled by Kconfig.

No functional change intended.

Cc: Jon Kohler <jon@nutanix.com>
Reviewed-by: Jon Kohler <jon@nutanix.com>
Link: https://lore.kernel.org/r/20230602010550.785722-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>