tipc: remove ub->ubsock checks

Both tipc_udp_enable and tipc_udp_disable are called under rtnl_lock,
ub->ubsock could never be NULL in tipc_udp_disable and cleanup_bearer,
so remove the check.

Also remove the one in tipc_udp_enable by adding "free" label.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>