networking: make skb_push & __skb_push return void pointers

It seems like a historic accident that these return unsigned char *,
and in many places that means casts are required, more often than not.

Make these functions return void * and remove all the casts across
the tree, adding a (u8 *) cast only where the unsigned char pointer
was used directly, all done with the following spatch:

    @@
    expression SKB, LEN;
    typedef u8;
    identifier fn = { skb_push, __skb_push, skb_push_rcsum };
    @@
    - *(fn(SKB, LEN))
    + *(u8 *)fn(SKB, LEN)

    @@
    expression E, SKB, LEN;
    identifier fn = { skb_push, __skb_push, skb_push_rcsum };
    type T;
    @@
    - E = ((T *)(fn(SKB, LEN)))
    + E = fn(SKB, LEN)

    @@
    expression SKB, LEN;
    identifier fn = { skb_push, __skb_push, skb_push_rcsum };
    @@
    - fn(SKB, LEN)[0]
    + *(u8 *)fn(SKB, LEN)

Note that the last part there converts from push(...)[0] to the
more idiomatic *(u8 *)push(...).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/nfc/pn544/pn544.c

@@ -649,8 +649,8 @@ static int pn544_hci_im_transceive(struct nfc_hci_dev *hdev,
 		} else
 			return 1;
 	case PN544_RF_READER_F_GATE:
-		*skb_push(skb, 1) = 0;
-		*skb_push(skb, 1) = 0;
+		*(u8 *)skb_push(skb, 1) = 0;
+		*(u8 *)skb_push(skb, 1) = 0;
 
 		info->async_cb_type = PN544_CB_TYPE_READER_F;
 		info->async_cb = cb;
@@ -665,7 +665,7 @@ static int pn544_hci_im_transceive(struct nfc_hci_dev *hdev,
 					      PN544_JEWEL_RAW_CMD, skb->data,
 					      skb->len, cb, cb_context);
 	case PN544_RF_READER_NFCIP1_INITIATOR_GATE:
-		*skb_push(skb, 1) = 0;
+		*(u8 *)skb_push(skb, 1) = 0;
 
 		return nfc_hci_send_event(hdev, target->hci_reader_gate,
 					PN544_HCI_EVT_SND_DATA, skb->data,
@@ -680,7 +680,7 @@ static int pn544_hci_tm_send(struct nfc_hci_dev *hdev, struct sk_buff *skb)
 	int r;
 
 	/* Set default false for multiple information chaining */
-	*skb_push(skb, 1) = 0;
+	*(u8 *)skb_push(skb, 1) = 0;
 
 	r = nfc_hci_send_event(hdev, PN544_RF_READER_NFCIP1_TARGET_GATE,
 			       PN544_HCI_EVT_SND_DATA, skb->data, skb->len);