dm crypt: replaced #if defined with IS_ENABLED

IS_ENABLED(CONFIG_ENCRYPTED_KEYS) is true whether the option is built-in
or a module, so use it instead of #if defined checking for each
separately.

The other #if was to avoid a static function defined, but unused
warning. As we now always build the callsite when the function
is defined, we can remove that first #if guard.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Acked-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>