Revert "prohibit rcutorture from being compiled into the kernel"

This reverts commit 9aaffc898ff4a3df18c5fc4b9e0fa47e779ad726.

That commit was a very bad idea. RCU_TORTURE found many boot timing
bugs and other sorts of bugs in the past, so excluding it from
boot images is very silly.

The option already depends on DEBUG_KERNEL and is disabled by default.
Even when it runs, the test threads are reniced. If it annoys people
we could add a runtime sysctl.

lib/Kconfig.debug

@@ -531,13 +531,14 @@ config BOOT_PRINTK_DELAY
 config RCU_TORTURE_TEST
 	tristate "torture tests for RCU"
 	depends on DEBUG_KERNEL
-	depends on m
 	default n
 	help
 	  This option provides a kernel module that runs torture tests
 	  on the RCU infrastructure.  The kernel module may be built
 	  after the fact on the running kernel to be tested, if desired.
 
+	  Say Y here if you want RCU torture tests to start automatically
+	  at boot time (you probably don't).
 	  Say M if you want the RCU torture tests to build as a module.
 	  Say N if you are unsure.