cpumask: only allocate nr_cpumask_bits.

Now we'll find out the hard way if anyone has CPUMASK_OFFSTACK and is
returning these or assigning them.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 646fade..4ad2d3c 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -11,6 +11,7 @@
 #include <linux/bitmap.h>
 #include <linux/bug.h>
 
+/* Don't assign or return these: may not be this big! */
 typedef struct cpumask { DECLARE_BITMAP(bits, NR_CPUS); } cpumask_t;
 
 /**
@@ -609,9 +610,7 @@
  */
 static inline size_t cpumask_size(void)
 {
-	/* FIXME: Once all cpumask assignments are eliminated, this
-	 * can be nr_cpumask_bits */
-	return BITS_TO_LONGS(NR_CPUS) * sizeof(long);
+	return BITS_TO_LONGS(nr_cpumask_bits) * sizeof(long);
 }
 
 /*