dpaa_eth: avoid uninitialized variable false-positive warning

We can now build this driver on ARM, so I ran into a randconfig build
warning that presumably had existed on powerpc already.

drivers/net/ethernet/freescale/dpaa/dpaa_eth.c: In function 'sg_fd_to_skb':
drivers/net/ethernet/freescale/dpaa/dpaa_eth.c:1712:18: error: 'skb' may be used uninitialized in this function [-Werror=maybe-uninitialized]

I'm slightly changing the logic here, to make it obvious to the
compiler that 'skb' is always initialized.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/dpaa/dpaa_eth.c

@@ -1721,6 +1721,7 @@ static struct sk_buff *sg_fd_to_skb(const struct dpaa_priv *priv,
 
 	/* Iterate through the SGT entries and add data buffers to the skb */
 	sgt = vaddr + fd_off;
+	skb = NULL;
 	for (i = 0; i < DPAA_SGT_MAX_ENTRIES; i++) {
 		/* Extension bit is not supported */
 		WARN_ON(qm_sg_entry_is_ext(&sgt[i]));
@@ -1738,7 +1739,7 @@ static struct sk_buff *sg_fd_to_skb(const struct dpaa_priv *priv,
 		count_ptr = this_cpu_ptr(dpaa_bp->percpu_count);
 		dma_unmap_single(dpaa_bp->dev, sg_addr, dpaa_bp->size,
 				 DMA_FROM_DEVICE);
-		if (i == 0) {
+		if (!skb) {
 			sz = dpaa_bp->size +
 				SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 			skb = build_skb(sg_vaddr, sz);