KVM: s390: remove _bh locking from start_stop_lock

The start_stop_lock is no longer acquired when in atomic context, therefore we
can convert it into an ordinary spin_lock.

Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>