selftests/bpf: Fix warning comparing pointer to 0

Avoid pointer type value compared with 0 to make code clear. Reported by
coccicheck:

  tools/testing/selftests/bpf/progs/map_ptr_kern.c:370:21-22:
  WARNING comparing pointer to 0
  tools/testing/selftests/bpf/progs/map_ptr_kern.c:397:21-22:
  WARNING comparing pointer to 0

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/1648605588-19269-1-git-send-email-baihaowen@meizu.com

tools/testing/selftests/bpf/progs/map_ptr_kern.c

@@ -367,7 +367,7 @@ static inline int check_array_of_maps(void)
 
 	VERIFY(check_default(&array_of_maps->map, map));
 	inner_map = bpf_map_lookup_elem(array_of_maps, &key);
-	VERIFY(inner_map != 0);
+	VERIFY(inner_map != NULL);
 	VERIFY(inner_map->map.max_entries == INNER_MAX_ENTRIES);
 
 	return 1;
@@ -394,7 +394,7 @@ static inline int check_hash_of_maps(void)
 
 	VERIFY(check_default(&hash_of_maps->map, map));
 	inner_map = bpf_map_lookup_elem(hash_of_maps, &key);
-	VERIFY(inner_map != 0);
+	VERIFY(inner_map != NULL);
 	VERIFY(inner_map->map.max_entries == INNER_MAX_ENTRIES);
 
 	return 1;