s390/dasd: fix inability to use DASD with DIAG driver

During initialization of the DASD DIAG driver a request is issued
that has a bio structure that resides on the stack. With virtually
mapped kernel stacks this bio address might be in virtual storage
which is unsuitable for usage with the diag250 call.
In this case the device can not be set online using the DIAG
discipline and fails with -EOPNOTSUP.
In the system journal the following error message is presented:

dasd: X.X.XXXX Setting the DASD online with discipline DIAG failed
with rc=-95

Fix by allocating the bio structure instead of having it on the stack.

Fixes: ce3dc447 ("s390: add support for virtually mapped kernel stacks")
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Cc: stable@vger.kernel.org #4.20
Signed-off-by: Jens Axboe <axboe@kernel.dk>