Fix a bug in kvm_mmu_get_page

A bug in kvm_mmu_page, when a kvm_mmu_page is allocated and re-walked by another
access, it will be ignored in for_each_gfn_sp(vcpu->kvm, sp, gfn), which should
be taken.

Remove some wired comments in __direct_map.

Signed-off-by: Arthur Chunqi Li <yzt356@gmail.com>