KVM: s390: Properly lock mm context allow_gmap_hpage_1m setting

We have to do down_write on the mm semaphore to set a bitfield in the
mm context.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Fixes: a4499382 ("KVM: s390: Add huge page enablement control")
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>