KVM: MMU: collapse TLB flushes when zap all pages

kvm_zap_obsolete_pages uses lock-break technique to zap pages,
it will flush tlb every time when it does lock-break

We can reload mmu on all vcpus after updating the generation
number so that the obsolete pages are not used on any vcpus,
after that we do not need to flush tlb when obsolete pages
are zapped

It will do kvm_mmu_prepare_zap_page many times and use one
kvm_mmu_commit_zap_page to collapse tlb flush, the side-effects
is that causes obsolete pages unlinked from active_list but leave
on hash-list, so we add the comment around the hash list walker

Note: kvm_mmu_commit_zap_page is still needed before free
the pages since other vcpus may be doing locklessly shadow
page walking

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>