iommu/riscv: prevent NULL deref in iova_to_phys

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.

Fixes: 488ffbf18171 ("iommu/riscv: Paging domain support")
Cc: Tomasz Jeznach <tjeznach@rivosinc.com>
Signed-off-by: XianLiang Huang <huangxianliang@lanxincomputing.com>
Link: https://lore.kernel.org/r/20250820072248.312-1-huangxianliang@lanxincomputing.com
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>