bug(dd): when iseek= is a very large number it panics

Hi, uutils mainteners

we confirmed dd panics when iseek= is a very large number

```rust
relunsec@relunsec:~/software/coreutils/target/debug$ ./dd if=a of=b iseek=166727721627616621762772

thread 'main' (112383) panicked at src/uu/dd/src/dd.rs:164:32:
attempt to multiply with overflow
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```