staging: lustre: remove ll_mrf_ret

It's just 'void' and only used in one place, so just remove it, it's
useless.

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