KVM: x86/mmu: Don't bottom out on leafs when zapping collapsible SPTEs

When zapping collapsible SPTEs in the TDP MMU, don't bottom out on a leaf
SPTE now that KVM doesn't require a PFN to compute the host mapping level,
i.e. now that there's no need to first find a leaf SPTE and then step
back up.

Drop the now unused tdp_iter_step_up(), as it is not the safest of
helpers (using any of the low level iterators requires some understanding
of the various side effects).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220715232107.3775620-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>