s390/dasd: fix error checks in dasd_copy_pair_store()

In the Linux kernel, the following vulnerability has been resolved: 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.