KVM: x86: cleanup kvm_inject_emulated_page_fault

To reconstruct the kvm_mmu to be used for page fault injection, we
can simply use fault->nested_page_fault.  This matches how
fault->nested_page_fault is assigned in the first place by
FNAME(walk_addr_generic).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>