ANDROID: iommu/io-pgtable-arm: Don't return on invalid ptes

The walker can be called for a range that contains invalid PTEs,
For example, with a table that has some already unmapped pages,
It's valid to unmap the whole table at once or even overwrite it
with a block, that will trigger flush_walk() operation for the
full table range which is not all valid.
So, the flush_walk() will walk the whole range and decrement the
refcount of the valid ptes which can be scattered.

Bug: 357781595
Bug: 384432312
Change-Id: If32bd389f29bbe5f6fedf1537cafc3f2badecc63
Signed-off-by: Mostafa Saleh <smostafa@google.com>