ANDROID: iommu/io-pgtable-arm: Fix block split for IO_PGTABLE_QUIRK_UNMAP_INVAL

When a block is split to a table because of a partial unmap,
a new table is populated, where the unmapped part are 0.

However, IO_PGTABLE_QUIRK_UNMAP_INVAL relies on the ability to
walk the tables after unmap to decrement their refcount, so
change the code to convert the block to a table, then unmap
the requested part.

Bug: 357781595
Bug: 384432312
Change-Id: I78be194b9af8e5454507d4b787232b84a38d41b7
Signed-off-by: Mostafa Saleh <smostafa@google.com>