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 <houtao1@huawei.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250315150930.1511727-1-houtao@huaweicloud.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>