KVM: MMU: make kvm_mmu_available_pages robust against n_used_mmu_pages > n_max_mmu_pages

As noticed by Ulrich Obergfell <uobergfe@redhat.com>, the mmu
counters are for beancounting purposes only - so n_used_mmu_pages and
n_max_mmu_pages could be relaxed (example: before f0f5933a),
resulting in n_used_mmu_pages > n_max_mmu_pages.

Make code robust against n_used_mmu_pages > n_max_mmu_pages.

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