net/sock: Update sk rcu iterator macro.

Mark hlist node in sk rcu iterator as protected by the rcu.
hlist_next_rcu accomplishes this and silences the warnings
sparse throws.

Found with make C=1 net/ipv4/udp.o on linux-next tag
next-20171009.

Signed-off-by: Tim Hansen <devtimhansen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>