vhost_vdpa: remove unnecessary spin_lock in vhost_vring_call

This commit removed unnecessary spin_locks in vhost_vring_call
and related operations. Because we manipulate irq offloading
contents in vhost_vdpa ioctl code path which is already
protected by dev mutex and vq mutex.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Link: https://lore.kernel.org/r/20200909065234.3313-1-lingshan.zhu@intel.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>