iommu/io-pgtable-fast: validate data before use

Return error on data allocation failure to prevent use
after free case.

Change-Id: I9d173756e277c9006220c02c209309167f179609
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>