KVM: s390: kvm_s390_gisa_clear() now clears the IPM only

Function kvm_s390_gisa_clear() now clears the Interruption
Pending Mask of the GISA asap. If the GISA is in the alert
list at this time it stays in the list but is removed by
process_gib_alert_list().

Signed-off-by: Michael Mueller <mimu@linux.ibm.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Message-Id: <20190131085247.13826-13-mimu@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>