RDS: fix rds_tcp_init() error path

RDS: fix rds_tcp_init() error path

If register_pernet_subsys() fails, we shouldn't try to call
unregister_pernet_subsys().

Fixes: 467fa15356 ("RDS-TCP: Support multiple RDS-TCP listen endpoints, one per netns.")
Cc: [email protected]
Cc: Sowmini Varadhan <[email protected]>
Cc: David S. Miller <[email protected]>
Signed-off-by: Vegard Nossum <[email protected]>
Acked-by: Sowmini Varadhan <[email protected]>
Acked-by: Santosh Shilimkar <[email protected]>
Signed-off-by: David S. Miller <[email protected]>