iommu/io-pgtable-arm: Rationalise size check

It makes little sense to only validate the requested size after we think
we've found a matching block size - making the check up-front is simple,
and far more logical than waiting to walk off the bottom of the table to
infer that we must have been passed a bogus size to start with.

We're missing an equivalent check on the unmap path, so add that as well
for consistency.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>