af_unix: call proto_unregister() in the error path in af_unix_init()

If register unix_stream_proto returns error, unix_dgram_proto needs
be unregistered.

Fixes: 94531cfc ("af_unix: Add unix_stream_proto for sockmap")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>