KVM: x86/mmu: Add a helper function to check if an SPTE needs atomic write

KVM: x86/mmu: Add a helper function to check if an SPTE needs atomic write

Move conditions in kvm_tdp_mmu_write_spte() to check if an SPTE should
be written atomically or not to a separate function.

This new function, kvm_tdp_mmu_spte_need_atomic_write(),  will be used
in future commits to optimize clearing bits in SPTEs.

Signed-off-by: Vipin Sharma <vipinsh@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Link: https://lore.kernel.org/r/20230321220021.2119033-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>