net: do not take an additional reference in skb_frag_set_page

I audited all of the callers in the tree and only one of them (pktgen) expects
it to do so. Taking this reference is pretty obviously confusing and error
prone.

In particular I looked at the following commits which switched callers of
(__)skb_frag_set_page to the skb paged fragment api:

6a930b9f cxgb3: convert to SKB paged frag API.
5dc3e196 myri10ge: convert to SKB paged frag API.
0e0634d2 vmxnet3: convert to SKB paged frag API.
86ee8130 virtionet: convert to SKB paged frag API.
4a22c4c9 sfc: convert to SKB paged frag API.
18324d69 cassini: convert to SKB paged frag API.
b061b39e benet: convert to SKB paged frag API.
b7b6a688 bnx2: convert to SKB paged frag API.
804cf14e net: xfrm: convert to SKB frag APIs
ea2ab693 net: convert core to skb paged frag APIs

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>