s390/dasd: fix read device characteristic with CONFIG_VMAP_STACK=y

The dasd_eckd_restore_device() function calls dasd_generic_read_dev_chars
with a temporary buffer on the stack. With CONFIG_VMAP_STACK=y this is
a vmalloc address but dasd_generic_restore_device() uses the address of
the buffer as I/O address. Circumvent this by using the already allocated
cqr->data buffer for the RDC data.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>