When clearing PUD or PMD entries the child page table (if any) is freed
and the PUD or PMD entry is then cleared. This result in a small race
condition window during which a free page table could be accessed by the
IPMMU.
Fix it by clearing and flushing the PUD or PMD entry before freeing the
child page table.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>