bpf: Extract program_returns_void() for clarity

bpf: Extract program_returns_void() for clarity

From: Eduard Zingerman <eddyz87@gmail.com>

The check_return_code function has explicit checks on whether
a program type can return void. Factor this logic out to reuse
it later for both main progs and subprogs.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260228184759.108145-3-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>