LU-17190 osc: account DIO in flight using server-side locking

LU-17190 osc: account DIO in flight using server-side locking

Add accounting for DIO (using server-side DLM extent locking) in
flight.
We can see from osc.*.rpc_stats to judge whether all I/O RPC slots,
which are limit by @max_rpcs_in_flight, are used out by DIOs.

Change-Id: I3d2c6e607d19037bb399c2bceb50e64826263469
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56325
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>