iommu/arm-smmu-v3: Properly handle the return value of arm_smmu_cmdq_build_cmd()

1. Build command CMD_SYNC cannot fail. So the return value can be ignored.
2. The arm_smmu_cmdq_build_cmd() almost never fails, the addition of
   "unlikely()" can optimize the instruction pipeline.
3. Check the return value in arm_smmu_cmdq_batch_add().

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20210818080452.2079-2-thunder.leizhen@huawei.com


Signed-off-by: Will Deacon <will@kernel.org>