bpf: Check map->record at the beginning of check_and_free_fields()

When there are no special fields in the map value, there is no need to
invoke bpf_obj_free_fields(). Therefore, checking the validity of
map->record in advance.

After the change, the benchmark result of the per-cpu update case in
map_perf_test increased by 40% under a 16-CPU VM.

Signed-off-by: Hou Tao <[email protected]>
Acked-by: Kumar Kartikeya Dwivedi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Hou Tao

kernel/bpf/hashtab.c

@@ -787,6 +787,9 @@ static int htab_lru_map_gen_lookup(struct bpf_map *map,
 static void check_and_free_fields(struct bpf_htab *htab,
 				  struct htab_elem *elem)
 {
+	if (IS_ERR_OR_NULL(htab->map.record))
+		return;
+
 	if (htab_is_percpu(htab)) {
 		void __percpu *pptr = htab_elem_get_ptr(elem, htab->map.key_size);
 		int cpu;