With CONFIG_CFI_CLANG enabled, the kernel strictly enforces that
indirect function calls use a function pointer type that matches the
target function. As bpf_testmod_ctx_release() signature differs from
the btf_dtor_kfunc_t pointer type used for the destructor calls in
bpf_obj_free_fields(), add a stub function with the correct type to
fix the type mismatch.
Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>