s390/dasd: fix possible buffer overflow in copy_pair_show

dasd_copy_relation->entry[] array might be accessed out of bounds if the
loop does not break.

Fixes: a91ff09d

 ("s390/dasd: add copy pair setup")
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Reviewed-by: Jan Hoeppner <hoeppner@linux.ibm.com>
Link: https://lore.kernel.org/r/20221123160719.3002694-5-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>