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/drivers/net/gtp.c b/drivers/net/gtp.c
index ca110cd..8e333a8 100644
--- a/drivers/net/gtp.c
+++ b/drivers/net/gtp.c
@@ -398,7 +398,7 @@ static inline void gtp0_push_header(struct sk_buff *skb, struct pdp_ctx *pctx)
 	int payload_len = skb->len;
 	struct gtp0_header *gtp0;
 
-	gtp0 = (struct gtp0_header *) skb_push(skb, sizeof(*gtp0));
+	gtp0 = skb_push(skb, sizeof(*gtp0));
 
 	gtp0->flags	= 0x1e; /* v0, GTP-non-prime. */
 	gtp0->type	= GTP_TPDU;
@@ -415,7 +415,7 @@ static inline void gtp1_push_header(struct sk_buff *skb, struct pdp_ctx *pctx)
 	int payload_len = skb->len;
 	struct gtp1_header *gtp1;
 
-	gtp1 = (struct gtp1_header *) skb_push(skb, sizeof(*gtp1));
+	gtp1 = skb_push(skb, sizeof(*gtp1));
 
 	/* Bits    8  7  6  5  4  3  2	1
 	 *	  +--+--+--+--+--+--+--+--+