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 <paulmck@linux.vnet.ibm.com>
Acked-by: Josh Triplett <josh@kernel.org>
Cc: "Randy.Dunlap" <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>