bpf: Introduce struct bpf_kfunc_meta

bpf: Introduce struct bpf_kfunc_meta

There is code duplication between add_kfunc_call() and
fetch_kfunc_meta() collecting information about a kfunc from BTF.

Introduce struct bpf_kfunc_meta to hold common kfunc BTF data and
implement fetch_kfunc_meta() to fill it in, instead of struct
bpf_kfunc_call_arg_meta directly.

Then use these in add_kfunc_call() and (new) fetch_kfunc_arg_meta()
functions, and fixup previous usages of fetch_kfunc_meta() to
fetch_kfunc_arg_meta().

Besides the code dedup, this change enables add_kfunc_call() to access
kfunc->flags.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260120222638.3976562-3-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>