drivers/block/loop.c: fix comment typo in loop_config_discard

Discard requests are ignored if the encryption is enabled for the given
loop device.  Update comment to match the code, and similar comments
elsewhere in the file.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>