staging/lustre: Fix max_dirty_mb output in sysfs

%ul definitely was supposed to be %lu in the format string,
so we print long unsigned int value, not just unsigned int
with a letter l added at the end.

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