rcu: Disable TASKS_RCU for usermode Linux

Usermode Linux currently does not implement arch_irqs_disabled_flags(),
which results in a build failure in TASKS_RCU.  Therefore, this commit
disables the TASKS_RCU Kconfig option in usermode Linux builds.  The
usermode Linux maintainers expect to merge arch_irqs_disabled_flags()
into 4.8, at which point this commit may be reverted.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Jeff Dike <jdike@addtoit.com>
Acked-by: Richard Weinberger <richard@nod.at>

init/Kconfig

@@ -517,6 +517,7 @@ config SRCU
 config TASKS_RCU
 	bool
 	default n
+	depends on !UML
 	select SRCU
 	help
 	  This option enables a task-based RCU implementation that uses