net: remove __skb_frag_set_page()

The remaining users calling __skb_frag_set_page() with
page being NULL seems to be doing defensive programming,
as shinfo->nr_frags is already decremented, so remove
them.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>