LU-5073 ptlrpc: unlink request buffer correctly

LU-5073 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>
Change-Id: I74e59e35a1c4b0c02a3ffa2db8c788c84683b7d6
Xyratex-bug-id: MRP-1848
Reviewed-on: http://review.whamcloud.com/10353
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>