Move CONFIG_DEVKMEM default to n

Kconfig comment suggests setting it as "n" if in doubt thus move the
default value to 'n'.

Signed-off-by: Dave Young <dyoung@redhat.com>
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>