KVM: x86/mmu: Dedup logic for detecting TLB flushes on leaf SPTE changes

Now that the shadow MMU and TDP MMU have identical logic for detecting
required TLB flushes when updating SPTEs, move said logic to a helper so
that the TDP MMU code can benefit from the comments that are currently
exclusive to the shadow MMU.

No functional change intended.

Link: https://lore.kernel.org/r/20241011021051.1557902-16-seanjc@google.com


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