staging/lustre/obdclass: export debugfs functionality for GPL only.

Turns out we mistakenly export some pretty-wide-reaching debugfs
functions as EXPORT_SYMBOL instead of EXPORT_SYMBOL_GPL as we should,
so this patch rectifies the situation.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>