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/net/wireless/ath/ar5523/ar5523.c

@@ -829,8 +829,8 @@ static void ar5523_tx_work_locked(struct ar5523 *ar)
 		data->ar = ar;
 		data->urb = urb;
 
-		desc = (struct ar5523_tx_desc *)skb_push(skb, sizeof(*desc));
-		chunk = (struct ar5523_chunk *)skb_push(skb, sizeof(*chunk));
+		desc = skb_push(skb, sizeof(*desc));
+		chunk = skb_push(skb, sizeof(*chunk));
 
 		chunk->seqnum = 0;
 		chunk->flags = UATH_CFLAGS_FINAL;