linuxkpi: Avoid trailing whitespaces in lkpi_hex_dump()

[dumbbell]

Summary

We use the return value of the callback to track the number of bytes written. We use it to determine if a group of characters should be prepended with a whitespace. This way, we never add a trailing whitespace.

We need to pay attention to the return value of the callback: if it is negative, it's an error and we return immediately. Otherwise, we would decrease the number of written bytes and possibly make it negative.

Sponsored by: The FreeBSD Foundation

[bz]

As pointed out in the other review; the callback functions can return negative values; you need to catch those and make sure you do not go negative or backwards.

[dumbbell]

Handle negative return value from callback.