iommu: arm-smmu: Don't call iommu unmap with zero size

If guard pages are disabled, don't call iommu_unmap for them.
Additionally, ensure the iova passed to __fast_smmu_free_iova()
is properly aligned.

Change-Id: I26cb6b354deb8a15458e5b7bf704c9c77d1c24ec
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>