iommu: Stop exporting free_iova_mem()

It has no user outside iova.c

Signed-off-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/1607020492-189471-4-git-send-email-john.garry@huawei.com


Signed-off-by: Will Deacon <will@kernel.org>