iommu/iova: Consolidate flush queue code

Squash and simplify some of the freeing code, and move the init
and free routines down into the rest of the flush queue code to
obviate the forward declarations.

Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/b0dd4565e6646b6489599d7a1eaa362c75f53c95.1639753638.git.robin.murphy@arm.com


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