iommu/io-pgtable-arm: Ensure we free the final level on teardown

When tearing down page tables, we return early for the final level
since we know that we won't have any table pointers to follow.
Unfortunately, this also means that we forget to free the final level,
so we end up leaking memory.

Fix the issue by always freeing the current level, but just don't bother
to iterate over the ptes if we're at the final level.

Cc: <stable@vger.kernel.org>
Reported-by: Zhang Bo <zhangbo_a@xiaomi.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>