scftorture: Add smp_call_function() torture test

This commit adds an smp_call_function() torture test that repeatedly
invokes this function and complains if things go badly awry.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
diff --git a/kernel/Makefile b/kernel/Makefile
index 9a20016..c45f551 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -133,6 +133,8 @@
 KCSAN_SANITIZE_stackleak.o := n
 KCOV_INSTRUMENT_stackleak.o := n
 
+obj-$(CONFIG_SCF_TORTURE_TEST) += scftorture.o
+
 $(obj)/configs.o: $(obj)/config_data.gz
 
 targets += config_data.gz