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>

lib/Kconfig

@@ -157,4 +157,11 @@ config CHECK_SIGNATURE
 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.
+
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