s390/dasd: make query host access interruptible

If the DASD device gets blocked for any reason, e.g. because it is reserved
somewhere, the host_access_count sysfs entry or the host_access_list
debugfs entry may sleep forever. Make it interruptible so that userspace
can use ^C to abort the operation.

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