Re-introduce zapping only leaf SPTEs in kvm_zap_gfn_range() and
kvm_tdp_mmu_unmap_gfn_range(), this time without losing a pending TLB
flush when processing multiple roots (including nested TDP shadow roots).
Dropping the TLB flush resulted in random crashes when running Hyper-V
Server 2019 in a guest with KSM enabled in the host (or any source of
mmu_notifier invalidations, KSM is just the easiest to force).

This effectively revert commits 873dd122
and fcb93eb6, and thus restores commit
cf3e2642, plus this delta on top:

bool kvm_tdp_mmu_zap_leafs(struct kvm *kvm, int as_id, gfn_t start, gfn_t end,
        struct kvm_mmu_page *root;

        for_each_tdp_mmu_root_yield_safe(kvm, root, as_id)
-               flush = tdp_mmu_zap_leafs(kvm, root, start, end, can_yield, false);
+               flush = tdp_mmu_zap_leafs(kvm, root, start, end, can_yield, flush);

        return flush;
 }

Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20220325230348.2587437-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>