kvm: x86: mmu: Refactor accessed/dirty checks in mmu_spte_update/clear

This simplifies mmu_spte_update() a little bit.
The checks for clearing of accessed and dirty bits are refactored into
separate functions, which are used inside both mmu_spte_update() and
mmu_spte_clear_track_bits(), as well as kvm_test_age_rmapp(). The new
helper functions handle both the case when A/D bits are supported in
hardware and the case when they are not.

Signed-off-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>