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/st/cw1200/txrx.c

@@ -574,7 +574,7 @@ cw1200_tx_h_wsm(struct cw1200_common *priv,
 		return NULL;
 	}
 
-	wsm = (struct wsm_tx *)skb_push(t->skb, sizeof(struct wsm_tx));
+	wsm = skb_push(t->skb, sizeof(struct wsm_tx));
 	t->txpriv.offset += sizeof(struct wsm_tx);
 	memset(wsm, 0, sizeof(*wsm));
 	wsm->hdr.len = __cpu_to_le16(t->skb->len);