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.