cpumask: Add CONFIG_CPUMASK_OFFSTACK

Impact: Add config option to enable code in cpumask.h

Currently it can be set if DEBUG_PER_CPU_MAPS, or set specifically by
an arch.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/lib/Kconfig b/lib/Kconfig
index 85cf7ea..7823f83 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -157,4 +157,11 @@
 config HAVE_LMB
 	boolean
 
+config CPUMASK_OFFSTACK
+	bool "Force CPU masks off stack" if DEBUG_PER_CPU_MAPS
+	help
+	  Use dynamic allocation for cpumask_var_t, instead of putting
+	  them on the stack.  This is a bit more expensive, but avoids
+	  stack overflow.
+
 endmenu