[NETFILTER]: nf_sockopts list head cleanup

Code is using knowledge that nf_sockopt_ops::list list_head is first
field in structure by using casts. Switch to list_for_each_entry()
itetators while I am at it.

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>