kmsg-reader: Seek to the end of the ringbuffer

kmsg-reader: Seek to the end of the ringbuffer

Otherwise, whenever plymouth starts we'd replay all previous kmsg
entries, even if they've already been logged to the console. This leads
to duplicated log entires, and makes it hard to debug things.

With /dev/console, we only log what we capture while Plymouth is
running. Let's do the same with /dev/kmsg