iommu/arm-smmu: Fix missing unlock on error in arm_smmu_device_group()

Add the missing unlock before return from function arm_smmu_device_group()
in the error handling case.

Fixes: b1a13479

 ("iommu/arm-smmu: Fix race condition during iommu_group creation")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210820074949.1946576-1-yangyingliang@huawei.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>