iommu/riscv: prevent NULL deref in iova_to_phys

In the Linux kernel, the following vulnerability has been resolved: iommu/riscv: prevent NULL deref in iova_to_phys The riscv_iommu_pte_fetch() function returns either NULL for unmapped/never-mapped iova, or a valid leaf pte pointer that requires no further validation. riscv_iommu_iova_to_phys() failed to handle NULL returns. Prevent null pointer dereference in riscv_iommu_iova_to_phys(), and remove the pte validation.