KVM: replace x86 kvm n_free_mmu_pages with n_used_mmu_pages

Doing this makes the code much more readable.  That's
borne out by the fact that this patch removes code.  "used"
also happens to be the number that we need to return back to
the slab code when our shrinker gets called.  Keeping this
value as opposed to free makes the next patch simpler.

So, 'struct kvm' is kzalloc()'d.  'struct kvm_arch' is a
structure member (and not a pointer) of 'struct kvm'.  That
means they start out zeroed.  I _think_ they get initialized
properly by kvm_mmu_change_mmu_pages().  But, that only happens
via kvm ioctls.

Another benefit of storing 'used' intead of 'free' is
that the values are consistent from the moment the structure is
allocated: no negative "used" value.

Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Tim Pepper <lnxninja@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>