iommu/riscv: Remove overflows on the invalidation path

In the Linux kernel, the following vulnerability has been resolved:

iommu/riscv: Remove overflows on the invalidation path

Since RISC-V supports a sign extended page table it should support

a gather->end of ULONG_MAX, but if this happens it will infinite loop

because of the overflow.

Also avoid overflow computing the length by moving the +1 to the other

side of the <