KVM: x86/mmu: Explicitly track only a single invalid mmu generation

KVM: x86/mmu: Explicitly track only a single invalid mmu generation

Toggle mmu_valid_gen between '0' and '1' instead of blindly incrementing
the generation.  Because slots_lock is held for the entire duration of
zapping obsolete pages, it's impossible for there to be multiple invalid
generations associated with shadow pages at any given time.

Toggling between the two generations (valid vs. invalid) allows changing
mmu_valid_gen from an unsigned long to a u8, which reduces the size of
struct kvm_mmu_page from 160 to 152 bytes on 64-bit KVM, i.e. reduces
KVM's memory footprint by 8 bytes per shadow page.

Set sp->mmu_valid_gen before it is added to active_mmu_pages.
Functionally this has no effect as kvm_mmu_alloc_page() has a single
caller that sets sp->mmu_valid_gen soon thereafter, but visually it is
jarring to see a shadow page being added to the list without its
mmu_valid_gen first being set.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>