staging/lustre/llite: Move unstable_stats from sysfs to debugfs

It's multiple values per file, so it has no business being in sysfs,
besides it was assuming seqfile anyway.

Fixes: d806f30e ("staging: lustre: osc: revise unstable pages accounting")
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>