bpf, btf: Check btf for register_bpf_struct_ops

bpf, btf: Check btf for register_bpf_struct_ops

Similar to the handling in the functions __register_btf_kfunc_id_set()
and register_btf_id_dtor_kfuncs(), this patch uses the newly added
helper check_btf_kconfigs() to handle module with its btf section
stripped.

While at it, the patch also adds the missed IS_ERR() check to fix the
commit f6be98d19985 ("bpf, net: switch to dynamic registration")

Fixes: f6be98d19985 ("bpf, net: switch to dynamic registration")
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/69082b9835463fe36f9e354bddf2d0a97df39c2b.1707373307.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>