net: atlantic: fix "frag[0] not initialized"

In aq_ring_rx_clean(), if buff->is_eop is not set AND
buff->len < AQ_CFG_RX_HDR_SIZE, then hdr_len remains equal to
buff->len and skb_add_rx_frag(xxx, *0*, ...) is not called.

The loop following this code starts calling skb_add_rx_frag() starting
with i=1 and thus frag[0] is never initialized. Since i is initialized
to zero at the top of the primary loop, we can just reference and
post-increment i instead of hardcoding the 0 when calling
skb_add_rx_frag() the first time.
Reported-by: Aashay Shringarpure <aashay@google.com>
Reported-by: Yi Chou <yich@google.com>
Reported-by: Shervin Oloumi <enlightened@google.com>
Signed-off-by: Grant Grundler <grundler@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/aquantia/atlantic/aq_ring.c

@@ -446,7 +446,7 @@ int aq_ring_rx_clean(struct aq_ring_s *self,
 		       ALIGN(hdr_len, sizeof(long)));
 
 		if (buff->len - hdr_len > 0) {
-			skb_add_rx_frag(skb, 0, buff->rxdata.page,
+			skb_add_rx_frag(skb, i++, buff->rxdata.page,
 					buff->rxdata.pg_off + hdr_len,
 					buff->len - hdr_len,
 					AQ_CFG_RX_FRAME_MAX);
@@ -455,7 +455,6 @@ int aq_ring_rx_clean(struct aq_ring_s *self,
 
 		if (!buff->is_eop) {
 			buff_ = buff;
-			i = 1U;
 			do {
 				next_ = buff_->next;
 				buff_ = &self->buff_ring[next_];