kfmt: fix bug where calls to copy() resulted in garbage being copied
kfmt: fix bug where calls to copy() resulted in garbage being copied

The bug manifested itself as a series of null bytes appearing to the
tty device when the ring buffer gets linked to it and its contents are
flushed to the tty.

Using gdb, I was able to track the problem into the ringbuf's Read
method and specifically to the call to copy(). I have replaced the call
with a for loop that the compiler would anyway optimize into a rep stosb
or equivalent asm instruction.

achilleasa/gopher-os,achilleasa/gopher-os