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.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>