vhost: vdpa: remove unnecessary null check

container_of is never null, so this null check is
unnecessary.

Addresses-Coverity-ID: 1492006 ("Logically dead code")
Fixes: 20453a45fb06 ("vhost: introduce vDPA-based backend")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200330235040.GA9997@embeddedor


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>