LU-14834: list the UUIDs of stale clients during recovery

LU-14834: list the UUIDs of stale clients during recovery

Add a new entry in debugfs for MDT and OSD to display the uuid of clients
yet to be reconnected during a recovery.

For example:

$ lctl get_param obdfilter.lustre-OST0000.recovery_stale_clients
obdfilter.lustre-OST0000.stale_clients=
9a7ab21d-207c-4680-b9bf-b5873fd05540

This will display, during the recovery, the UUIDs of clients
that are expected to be connected.

Signed-off-by: Courrier Guillaume <guillaume.courrier@cea.fr>
Change-Id: Ib8c0b500adc9098e3cfb1998df06757a7d31b7b9
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/44196
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>