s390/dasd: fix error checks in dasd_copy_pair_store()

dasd_add_busid() can return an error via ERR_PTR() if an allocation
fails. However, two callsites in dasd_copy_pair_store() do not check
the result, potentially resulting in a NULL pointer dereference. Fix
this by checking the result with IS_ERR() and returning the error up
the stack.

Fixes: a91ff09d

 ("s390/dasd: add copy pair setup")
Signed-off-by: Carlos López <clopez@suse.de>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://lore.kernel.org/r/20240715112434.2111291-3-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>