ping: Convert hlist_nulls to plain hlist.

Since introduced in commit c319b4d7 ("net: ipv4: add IPPROTO_ICMP
socket kind"), ping socket does not use SLAB_TYPESAFE_BY_RCU nor check
nulls marker in loops.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>