MRP-1579 ptlrpc: fix for invalid unlock

Very rare situation should happened to hit this issue
req->rq_export != NULL and cfs_list_empty(req->rq_export_list).
Bug was introduced by this commit 3dcc3c01.

Signed-off-by: Alexander Boyko <alexander_boyko@xyratex.com>
Change-Id: Ic71e68ec1b17f74a9dd772f8a65e80b65529e381
Reviewed-on: http://es-gerrit.xyus.xyratex.com:8080/1287
Tested-by: Jenkins
Reviewed-by: Andriy Skulysh <andriy_skulysh@xyratex.com>
Tested-by: Elena Gryaznova <elena_gryaznova@xyratex.com>
Reviewed-by: Vitaly Fertman <vitaly_fertman@xyratex.com>