iommu/arm-smmu-v3: Add a check to avoid invalid iotlb sync

It may send a invalid tlb sync for smmuv3 if iotlb_gather is not valid
(iotlb_gather->pgsize = 0). So add a check to avoid invalid iotlb sync
for it.

Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Link: https://lore.kernel.org/r/1617109106-121844-1-git-send-email-chenxiang66@hisilicon.com
Signed-off-by: Will Deacon <will@kernel.org>