virtio: rng: allow tasks to be killed that are waiting for rng input

Use wait_for_completion_killable() instead of wait_for_completion() when
waiting for the host to send us entropy.  Without this,

  # cat /dev/hwrng
  ^C

just hangs.
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/char/hw_random/virtio-rng.c

@@ -55,6 +55,7 @@ static void register_buffer(u8 *buf, size_t size)
 
 static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 {
+	int ret;
 
 	if (!busy) {
 		busy = true;
@@ -65,7 +66,9 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 	if (!wait)
 		return 0;
 
-	wait_for_completion(&have_data);
+	ret = wait_for_completion_killable(&have_data);
+	if (ret < 0)
+		return ret;
 
 	busy = false;