[PATCH -next] virtio: balloon: fix missing unlock on error in fill_balloon()

From: Wei Yongjun <redacted>

Add the missing unlock before return from function fill_balloon()
in the error handling case.

Introduced by 9864a8(virtio_balloon: introduce migration primitives
to balloon pages)

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch)

Signed-off-by: Wei Yongjun <redacted>
---
 drivers/virtio/virtio_balloon.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index f70151b..72e8dcb 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c

@@ -152,8 +152,10 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
 	}
 
 	/* Didn't get any?  Oh well. */
-	if (vb->num_pfns == 0)
+	if (vb->num_pfns == 0) {
+		mutex_unlock(&vb->balloon_lock);
 		return;
+	}
 
 	tell_host(vb, vb->inflate_vq);
 	mutex_unlock(&vb->balloon_lock);