Combine the back-to-back if-statements for synchronizing children when
linking a new indirect shadow page in order to decrease the indentation,
and to make it easier to "see" the logic in its entirety.

No functional change intended.

Link: https://lore.kernel.org/r/20240802203900.348808-2-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>