iommu: Stop exporting free_iova_fast()

Function free_iova_fast() is only referenced by dma-iommu.c, which can
only be in-built, so stop exporting it.

This was missed in an earlier tidy-up patch.

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/1616675401-151997-5-git-send-email-john.garry@huawei.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>