staging/lustre/ptlrpc: unlink request buffer correctly

outgoning buffer may be hold by lnet and don't unlinked fast,
it's break unloading a lustre modules as request hold a
reference to the export/obd

Signed-off-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Xyratex-bug-id: MRP-1848
Reviewed-on: http://review.whamcloud.com/10353
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5073


Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-by: Isaac Huang <he.huang@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>