LU-11289 ptlrpc: fix ASSERTION on scp_rqbd_posted

LU-11289 ptlrpc: fix ASSERTION on scp_rqbd_posted

The request may be referenced by other target even the threads
of service were stopped. It caused by some portal shared among
different services. Just wait the request to be released as a
workaround.

LustreError: (service.c::ptlrpc_service_purge_all())
	ASSERTION( list_empty(&svcpt->scp_rqbd_posted) ) failed:
LustreError: (service.c::ptlrpc_service_purge_all()) LBUG
Pid: 21, comm: umount 3.10.0 #1 SMP
Call Trace:
 [<a01c47dc>] libcfs_call_trace+0x8c/0xc0 [libcfs]
 [<a01c488c>] lbug_with_loc+0x4c/0xa0 [libcfs]
 [<a0b534dd>] ptlrpc_unregister_service+0xced/0xd90 [ptlrpc]
 [<a005e122>] ost_cleanup+0x82/0x1b0 [ost]
 [<a08e0bfa>] class_free_dev+0x1ca/0x630 [obdclass]
 [<a08e1240>] class_export_put+0x1e0/0x2b0 [obdclass]
 [<a08e2cc5>] class_unlink_export+0x135/0x170 [obdclass]
 [<a08f8030>] class_decref+0x80/0x160 [obdclass]
 [<a08f8481>] class_detach+0x1b1/0x2e0 [obdclass]
 [<a08fef21>] class_process_config+0x1a91/0x2820 [obdclass]
 [<a08ffe90>] class_manual_cleanup+0x1e0/0x6d0 [obdclass]
 [<a092a115>] server_stop_servers+0xd5/0x160 [obdclass]
 [<a092f6c6>] server_put_super+0x126/0xca0 [obdclass]
 [<8121068a>] generic_shutdown_super+0x6a/0xf0
 [<81210a62>] kill_anon_super+0x12/0x20
 [<a09027e2>] lustre_kill_super+0x32/0x50 [obdclass]
 [<81210e59>] deactivate_locked_super+0x49/0x60
 [<812115a6>] deactivate_super+0x46/0x60
 [<8123019f>] cleanup_mnt+0x3f/0x80
 [<81230232>] __cleanup_mnt+0x12/0x20
 [<810ab085>] task_work_run+0xb5/0xf0
 [<8102ac12>] do_notify_resume+0x92/0xb0
 [<81783c83>] int_signal+0x12/0x17
 Kernel panic - not syncing: LBUG

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: Idfb19df123ceae177a0e447e9344bac6861166bf
Reviewed-on: https://review.whamcloud.com/41936
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>