iommu/riscv: Fixup compile warning

iommu/riscv: Fixup compile warning

When __BITS_PER_LONG == 32, size_t is defined as unsigned int rather
than unsigned long. Therefore, we should use size_t to avoid
type-checking errors.

Fixes: 488ffbf18171 ("iommu/riscv: Paging domain support")
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Tomasz Jeznach <tjeznach@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Tomasz Jeznach <tjeznach@rivosinc.com>
Link: https://lore.kernel.org/r/20250103024616.3359159-1-guoren@kernel.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>