ping_lookup() does not acquire the table spinlock, so iteration should
use hlist_nulls_for_each_entry_rcu().

Spotted during code review.

Fixes: dbca1596 ("ping: convert to RCU lookups, get rid of rwlock")
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20221129140644.28525-1-fw@strlen.deSigned-off-by: Paolo Abeni <pabeni@redhat.com>