iommu/arm-smmu-v3: Fix a NULL vs IS_ERR() check

The arm_smmu_domain_alloc() function returns error pointers on error.  It
doesn't return NULL.  Update the error checking to match.

Fixes: 52acd7d8 ("iommu/arm-smmu-v3: Add support for domain_alloc_user fn")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/9208cd0d-8105-40df-93e9-bdcdf0d55eec@stanley.mountain


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