x86/kvm: Cache gfn to pfn translation

x86/kvm: Cache gfn to pfn translation

__kvm_map_gfn()'s call to gfn_to_pfn_memslot() is
* relatively expensive
* in certain cases (such as when done from atomic context) cannot be called

Stashing gfn-to-pfn mapping should help with both cases.

This is part of CVE-2019-3016.

Signed-off-by: Boris Ostrovsky <[email protected]>
Reviewed-by: Joao Martins <[email protected]>
Cc: [email protected]
Signed-off-by: Paolo Bonzini <[email protected]>