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

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.com
Signed-off-by: Sean Christopherson <seanjc@google.com>