prohibit rcutorture from being compiled into the kernel

There have been a number of instances where people have accidentally compiled
rcutorture into the kernel (CONFIG_RCU_TORTURE_TEST=y), which has never been
useful, and has often resulted in great frustration.

The attached patch prohibits rcutorture from being compiled into the
kernel. It may be excluded altogether or compiled as a module. People
wishing to have rcutorture hammer their machine immediately upon boot
are free to hand-edit lib/Kconfig.debug to remove the "depends on m"
line.

Thanks to Randy Dunlap for the trick that makes this work.

Signed-off-by: Paul E. McKenney <[e-mail]>
Acked-by: Josh Triplett <[e-mail]>
Cc: "Randy.Dunlap" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/Kconfig.debug

···
 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.