networking: make skb_pull & friends 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_pull,
            __skb_pull,
            skb_pull_inline,
            __pskb_pull_tail,
            __pskb_pull,
            pskb_pull
    };
    @@
    - *(fn(SKB, LEN))
    + *(u8 *)fn(SKB, LEN)

    @@
    expression E, SKB, LEN;
    identifier fn = {
            skb_pull,
            __skb_pull,
            skb_pull_inline,
            __pskb_pull_tail,
            __pskb_pull,
            pskb_pull
    };
    type T;
    @@
    - E = ((T *)(fn(SKB, LEN)))
    + E = fn(SKB, LEN)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>