block: cryptoloop - Fix build warning

This patch fix the following build warning:
drivers/block/cryptoloop.c:46:8: warning: variable 'cipher' set but not used [-Wunused-but-set-variable]

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>