KVM: x86/mmu: Properly dereference rcu-protected TDP MMU sptep iterator

Wrap the read of iter->sptep in tdp_mmu_map_handle_target_level() with
rcu_dereference().  Shadow pages in the TDP MMU, and thus their SPTEs,
are protected by rcu.

This fixes a Sparse warning at tdp_mmu.c:900:51:
  warning: incorrect type in argument 1 (different address spaces)
  expected unsigned long long [usertype] *sptep
  got unsigned long long [noderef] [usertype] __rcu *[usertype] sptep

Fixes: 7158bee4

 ("KVM: MMU: pass kvm_mmu_page struct to make_spte")
Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211103161833.3769487-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>