intel-iommu: only do page walk for MAP notifiers
    
    For UNMAP-only IOMMU notifiers, we don't need to walk the page tables.
    Fasten that procedure by skipping the page table walk.  That should
    boost performance for UNMAP-only notifiers like vhost.
    
    CC: QEMU Stable <qemu-stable@nongnu.org>
    Signed-off-by: Peter Xu <peterx@redhat.com>
    Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
    Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
    (cherry picked from commit 4f8a62a933a79094e44bc1b16b63bb23e62d67b4)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>