s390/dasd: check blockdevice pointer before trying to sync blockdevice

If safe offline is called for a DASD alias device a null pointer is passed
to fsync_bdev. So check for existence of the blockdevice before calling
fsync_bdev.
Should not be a real world problem since safe offline for an alias device
does not make sense and fsync_bdev can deal with a NULL pointer which it
gets after successful NULL pointer dereferencing on s390.

Signed-off-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>