Revert "net: make get_net_ns return error if NET_NS is disabled"

hulk inclusion
category: bugfix
bugzilla: NA
CVE: NA

--------------------------------

The commit a7957d8c ("net: make get_net_ns return error...")
breaks the kabi, it fixes the bug only with CONFIG_NET_NS disabled,
the config is always enabled, so it can be revert.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>