iommu/vt-d: Fix mm reference leak

Move canonical address check before mmget_not_zero() to avoid mm
reference leak.

Fixes: 9d8c3af3 ("iommu/vt-d: IOMMU Page Request needs to check if address is canonical.")
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Acked-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>