selftests/bpf: Fix error return code in run_getsockopt_test()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 65b4414a ("selftests/bpf: add sockopt test that exercises BPF_F_ALLOW_MULTI")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201116101633.64627-1-wanghai38@huawei.com