KVM: PPC: Book3S HV: Hold kvm->mmu_lock across updating nested pte rc bits

We already hold the kvm->mmu_lock spin lock across updating the rc bits
in the pte for the L1 guest. Continue to hold the lock across updating
the rc bits in the pte for the nested guest as well to prevent
invalidations from occurring.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>