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>
diff --git a/net/sctp/ulpevent.c b/net/sctp/ulpevent.c
index e361f0b..17854fb 100644
--- a/net/sctp/ulpevent.c
+++ b/net/sctp/ulpevent.c
@@ -153,8 +153,7 @@
 		sctp_ulpevent_init(event, MSG_NOTIFICATION, skb->truesize);
 
 		/* Include the notification structure */
-		sac = (struct sctp_assoc_change *)
-			skb_push(skb, sizeof(struct sctp_assoc_change));
+		sac = skb_push(skb, sizeof(struct sctp_assoc_change));
 
 		/* Trim the buffer to the right length.  */
 		skb_trim(skb, sizeof(struct sctp_assoc_change) +
@@ -400,7 +399,7 @@
 	event = sctp_skb2event(skb);
 	sctp_ulpevent_init(event, MSG_NOTIFICATION, skb->truesize);
 
-	sre = (struct sctp_remote_error *) skb_push(skb, sizeof(*sre));
+	sre = skb_push(skb, sizeof(*sre));
 
 	/* Trim the buffer to the right length.  */
 	skb_trim(skb, sizeof(*sre) + elen);
@@ -451,8 +450,7 @@
 	event = sctp_skb2event(skb);
 	sctp_ulpevent_init(event, MSG_NOTIFICATION, skb->truesize);
 
-	ssf = (struct sctp_send_failed *)
-		skb_push(skb, sizeof(struct sctp_send_failed));
+	ssf = skb_push(skb, sizeof(struct sctp_send_failed));
 
 	/* Socket Extensions for SCTP
 	 * 5.3.1.4 SCTP_SEND_FAILED