kvm: x86: disable shattered huge page recovery for PREEMPT_RT.

If a huge page is recovered (and becomes no executable) while another
thread is executing it, the resulting contention on mmu_lock can cause
latency spikes.  Disabling recovery for PREEMPT_RT kernels fixes this
issue.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>