vduse: missing error code in vduse_init()

This should return -ENOMEM if alloc_workqueue() fails.  Currently it
returns success.

Fixes: b66219796563 ("vduse: Introduce VDUSE - vDPA Device in Userspace")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210907073223.GA18254@kili


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>