virtio_ring: fix return code on DMA mapping fails

Commit 780bc7903a ("virtio_ring: Support DMA APIs")  makes
virtqueue_add() return -EIO when we fail to map our I/O buffers. This is
a very realistic scenario for guests with encrypted memory, as swiotlb
may run out of space, depending on it's size and the I/O load.

The virtio-blk driver interprets -EIO form virtqueue_add() as an IO
error, despite the fact that swiotlb full is in absence of bugs a
recoverable condition.

Let us change the return code to -ENOMEM, and make the block layer
recover form these failures when virtio-blk encounters the condition
described above.

Cc: stable@vger.kernel.org
Fixes: 780bc7903a ("virtio_ring: Support DMA APIs")
Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Tested-by: Michael Mueller <mimu@linux.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_ring.c

@@ -583,7 +583,7 @@ unmap_release:
 		kfree(desc);
 
 	END_USE(vq);
-	return -EIO;
+	return -ENOMEM;
 }
 
 static bool virtqueue_kick_prepare_split(struct virtqueue *_vq)
@@ -1085,7 +1085,7 @@ unmap_release:
 	kfree(desc);
 
 	END_USE(vq);
-	return -EIO;
+	return -ENOMEM;
 }
 
 static inline int virtqueue_add_packed(struct virtqueue *_vq,