bpf: fixup error message from gpl helpers on license mismatch

Stating 'proprietary program' in the error is just silly since it
can also be a different open source license than that which is just
not compatible.

Reference: https://twitter.com/majek04/status/998531268039102465
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -2462,7 +2462,7 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
 
 	/* eBPF programs must be GPL compatible to use GPL-ed functions */
 	if (!env->prog->gpl_compatible && fn->gpl_only) {
-		verbose(env, "cannot call GPL only function from proprietary program\n");
+		verbose(env, "cannot call GPL-restricted function from non-GPL compatible program\n");
 		return -EINVAL;
 	}