Consolidate the direct MMU page fault handlers into a common helper,
direct_page_fault().  Except for unique max level conditions, the tdp
and nonpaging fault handlers are functionally identical.

No functional change intended.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>