ping: Fix potentail NULL deref for /proc/net/icmp.

In the Linux kernel, the following vulnerability has been resolved: ping: Fix potentail NULL deref for /proc/net/icmp. After commit dbca1596bbb0 ("ping: convert to RCU lookups, get rid of rwlock"), we use RCU for ping sockets, but we should use spinlock for /proc/net/icmp to avoid a potential NULL deref mentioned in the previous patch. Let's go back to using spinlock there. Note we can convert ping sockets to use hlist instead of hlist_nulls because we do not use SLAB_TYPESAFE_BY_RCU for ping sockets.