vhost: fix error handling in vring ioctls

Stanse found a locking problem in vhost_set_vring:
several returns from VHOST_SET_VRING_KICK, VHOST_SET_VRING_CALL,
VHOST_SET_VRING_ERR with the vq->mutex held.
Fix these up.

Reported-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Laurent Chavey <chavey@google.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -476,8 +476,10 @@ static long vhost_set_vring(struct vhost_dev *d, int ioctl, void __user *argp)
 		if (r < 0)
 			break;
 		eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
-		if (IS_ERR(eventfp))
-			return PTR_ERR(eventfp);
+		if (IS_ERR(eventfp)) {
+			r = PTR_ERR(eventfp);
+			break;
+		}
 		if (eventfp != vq->kick) {
 			pollstop = filep = vq->kick;
 			pollstart = vq->kick = eventfp;
@@ -489,8 +491,10 @@ static long vhost_set_vring(struct vhost_dev *d, int ioctl, void __user *argp)
 		if (r < 0)
 			break;
 		eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
-		if (IS_ERR(eventfp))
-			return PTR_ERR(eventfp);
+		if (IS_ERR(eventfp)) {
+			r = PTR_ERR(eventfp);
+			break;
+		}
 		if (eventfp != vq->call) {
 			filep = vq->call;
 			ctx = vq->call_ctx;
@@ -505,8 +509,10 @@ static long vhost_set_vring(struct vhost_dev *d, int ioctl, void __user *argp)
 		if (r < 0)
 			break;
 		eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
-		if (IS_ERR(eventfp))
-			return PTR_ERR(eventfp);
+		if (IS_ERR(eventfp)) {
+			r = PTR_ERR(eventfp);
+			break;
+		}
 		if (eventfp != vq->error) {
 			filep = vq->error;
 			vq->error = eventfp;