bpf: fix possible endless loop in BPF map iteration

The _safe variant used here gets the next element before running the callback,
avoiding the endless loop condition.

Signed-off-by: Brandon Kammerdiener <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>
Acked-by: Hou Tao <[e-mail]>

kernel/bpf/hashtab.c

···
 		b = &htab->buckets[i];
 		rcu_read_lock();
 		head = &b->head;
-		hlist_nulls_for_each_entry_rcu(elem, n, head, hash_node) {
+		hlist_nulls_for_each_entry_safe(elem, n, head, hash_node) {
 			key = elem->key;
 			if (is_percpu) {
 				/* current cpu value for percpu map */