KVM: x86/mmu: Rephrase comment about synthetic PFERR flags in #PF handler

Reword the BUILD_BUG_ON() comment in the legacy #PF handler to explicitly
describe how asserting that synthetic PFERR flags are limited to bits 31:0
protects KVM against inadvertently passing a synthetic flag to the common
page fault handler.

No functional change intended.
Suggested-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20240608001108.3296879-1-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/mmu.c

@@ -4552,7 +4552,10 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
 	if (WARN_ON_ONCE(error_code >> 32))
 		error_code = lower_32_bits(error_code);
 
-	/* Ensure the above sanity check also covers KVM-defined flags. */
+	/*
+	 * Restrict KVM-defined flags to bits 63:32 so that it's impossible for
+	 * them to conflict with #PF error codes, which are limited to 32 bits.
+	 */
 	BUILD_BUG_ON(lower_32_bits(PFERR_SYNTHETIC_MASK));
 
 	vcpu->arch.l1tf_flush_l1d = true;