KVM: x86: take slots_lock in kvm_free_pit

kvm_vm_release() did not have slots_lock when calling
kvm_io_bus_unregister_dev() and this went unnoticed until 4a12f951
("KVM: mark kvm->busses as rcu protected") added dynamic checks.
Luckily, there should be no race at that point:

  =============================
  WARNING: suspicious RCU usage
  4.12.0.kvm+ #0 Not tainted
  -----------------------------
  ./include/linux/kvm_host.h:479 suspicious rcu_dereference_check() usage!

   lockdep_rcu_suspicious+0xc5/0x100
   kvm_io_bus_unregister_dev+0x173/0x190 [kvm]
   kvm_free_pit+0x28/0x80 [kvm]
   kvm_arch_sync_events+0x2d/0x30 [kvm]
   kvm_put_kvm+0xa7/0x2a0 [kvm]
   kvm_vm_release+0x21/0x30 [kvm]

Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>