KVM: x86: make FNAME(fetch) and __direct_map more similar

These two functions are basically doing the same thing through
kvm_mmu_get_page, link_shadow_page and mmu_set_spte; yet, for historical
reasons, their code looks very different.  This patch tries to take the
best of each and make them very similar, so that it is easy to understand
changes that apply to both of them.

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