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/scsi/qedf/qedf_fip.c

@@ -125,8 +125,7 @@ void qedf_fip_send(struct fcoe_ctlr *fip, struct sk_buff *skb)
 	sub = fiph->fip_subcode;
 
 	if (!qedf->vlan_hw_insert) {
-		vlan_hdr = (struct vlan_ethhdr *)skb_push(skb, sizeof(*vlan_hdr)
-		    - sizeof(*eth_hdr));
+		vlan_hdr = skb_push(skb, sizeof(*vlan_hdr) - sizeof(*eth_hdr));
 		memcpy(vlan_hdr, eth_hdr, 2 * ETH_ALEN);
 		vlan_hdr->h_vlan_proto = htons(ETH_P_8021Q);
 		vlan_hdr->h_vlan_encapsulated_proto = eth_hdr->h_proto;