staging: lustre: remove RETURN macro

We have a kernel-wide function tracing system, so use that instead of
rolling a custom one just for one filesystem.

Cc: Peng Tao <tao.peng@emc.com>
Cc: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>