rcu: Re-enable TASKS_RCU for User Mode Linux

Now that User Mode Linux supports arch_irqs_disabled_flags(), this
commit re-enables TASKS_RCU for User Mode Linux.

Reported-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>