IS_ENABLED is almost always used with CONFIG_<FOO> defines.

Add a test to verify that the #define being tested starts with CONFIG_.
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/e7fda760b91b769ba82844ba282d432c0d26d709.camel@perches.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>