KVM: Documentation: Update fast page fault for indirect sp

We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.

For direct sp, we can easily avoid it since the spte of direct sp is fixed

to gfn. For indirect sp, before we do cmpxchg, we call gfn_to_pfn_atomic()

to pin gfn to pfn, because after gfn_to_pfn_atomic():

to gfn.  For indirect sp, we disabled fast page fault for simplicity.

A solution for indirect sp could be to pin the gfn, for example via

kvm_vcpu_gfn_to_pfn_atomic, before the cmpxchg.  After the pinning:

- We have held the refcount of pfn that means the pfn can not be freed and

  be reused for another gfn.

- The pfn is writable that means it can not be shared between different gfns

- The pfn is writable and therefore it cannot be shared between different gfns

  by KSM.

Then, we can ensure the dirty bitmaps is correctly set for a gfn.

Currently, to simplify the whole things, we disable fast page fault for

indirect shadow page.

2) Dirty bit tracking

In the origin code, the spte can be fast updated (non-atomically) if the