LU-16144 nrs: implement force mode for nrs_tbf_req_get()

LU-16144 nrs: implement force mode for nrs_tbf_req_get()

ptlrpc_service_purge_all() calls ptlrpc_server_request_get() with
"force=true" to purge all active requests before stopping an NRS
policy (when unregistering a service).

"force" mode should always return a request if a pending request is
present in the NRS policy.

nrs_tbf_req_get() does not implement such a mode and can return a
NULL pointer.
This can cause a crash when umounting a target if a TBF rule rate
threshold is reached:

BUG: unable to handle kernel NULL pointer dereference at
0000000000000114
IP: [<ffffffffc0d9e965>] ptlrpc_nrs_req_stop_nolock+0x5/0x150
.....
? ptlrpc_server_finish_active_request+0x2b/0x140 [ptlrpc]
ptlrpc_service_purge_all+0x137/0x920 [ptlrpc]
ptlrpc_unregister_service+0xe7/0x6f0 [ptlrpc]
ost_cleanup+0x52/0x1b0 [ost]
class_free_dev+0x21d/0x720 [obdclass]
class_export_put+0x1f0/0x2c0 [obdclass]
class_unlink_export+0x135/0x170 [obdclass]
class_decref+0x80/0x160 [obdclass]
class_detach+0x1b3/0x2e0 [obdclass]
class_process_config+0x1a38/0x2830 [obdclass]
? complete+0x4a/0x60
? list_del+0xd/0x30
? wait_for_completion+0x4e/0x140
class_manual_cleanup+0x1e0/0x710 [obdclass]
server_stop_servers+0xd5/0x160 [obdclass]
server_put_super+0x12d/0xd00 [obdclass]
generic_shutdown_super+0x6d/0x100

Signed-off-by: Etienne AUJAMES <etienne.aujames@cea.fr>
Change-Id: Ic4443700725d9308764fbf21cb7de6fa4ab41134
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48494
Reviewed-by: Nikitas Angelinas <nikitas.angelinas@hpe.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>