sfc: Use dev_kfree_skb() in efx_end_loopback()

Fix CID 102619 in the Coverity report on Linux.

efx_end_loopback() iterates over an array of skb pointers of which
some may be null (if efx_begin_loopback() failed).  It should not use
dev_kfree_skb_irq(), which requires non-null pointers.  In practice
this is safe because it does not run in interrupt context and
therefore always ends up calling dev_kfree_skb(), which does allow
null pointers.  But we should make that explicit.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

drivers/net/ethernet/sfc/selftest.c

@@ -488,7 +488,7 @@ static int efx_end_loopback(struct efx_tx_queue *tx_queue,
 		skb = state->skbs[i];
 		if (skb && !skb_shared(skb))
 			++tx_done;
-		dev_kfree_skb_any(skb);
+		dev_kfree_skb(skb);
 	}
 
 	netif_tx_unlock_bh(efx->net_dev);