KVM: Fix mmu shrinker error

kvm_mmu_remove_one_alloc_mmu_page() assumes kvm_mmu_zap_page() only reclaims
only one sp, but that's not the case. This will cause mmu shrinker returns
a wrong number. This patch fix the counting error.

Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>