bpf: Fix returned error sign when link doesn't support updates

System calls encode returned errors as negative values. Fix a typo that
breaks this convention for bpf(LINK_UPDATE) when bpf_link doesn't support
update operation.

Fixes: f9d04127 ("bpf: Refactor bpf_link update handling")
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200525122928.1164495-1-jakub@cloudflare.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/syscall.c

@@ -3924,7 +3924,7 @@ static int link_update(union bpf_attr *attr)
 	if (link->ops->update_prog)
 		ret = link->ops->update_prog(link, new_prog, old_prog);
 	else
-		ret = EINVAL;
+		ret = -EINVAL;
 
 out_put_progs:
 	if (old_prog)