KVM: x86/mmu: Drop level optimization from fast_page_fault()

Remove fast_page_fault()'s optimization to stop the shadow walk if the
iterator level drops below the intended map level.  The intended map
level is only acccurate for HugeTLB mappings (THP mappings are detected
after fast_page_fault()), i.e. it's not required for correctness, and
a future patch will also move HugeTLB mapping detection to after
fast_page_fault().

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>