KVM: Avoid atomic operations when kicking the running vCPU

If we do have the vcpu mutex, as is the case if kvm_running_vcpu is set
to the target vcpu of the kick, changes to vcpu->mode do not need atomic
operations; cmpxchg is only needed _outside_ the mutex to ensure that
the IN_GUEST_MODE->EXITING_GUEST_MODE change does not race with the vcpu
thread going OUTSIDE_GUEST_MODE.

Use this to optimize the case of a vCPU sending an interrupt to itself.

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