bpf: Add is_fd_htab() helper

Add is_fd_htab() helper to check whether the map is htab of maps.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20250401062250.543403-5-houtao@huaweicloud.com


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