LU-19091 ptlrpc: protect internal access to obd->obd_svc_stats

LU-19091 ptlrpc: protect internal access to obd->obd_svc_stats

PM-QoS patch from LU-18446, where OBD svc stats are used to
evaluate best time period for low CPUs latency to be kept, has
introduced a new and internal way to access obd->obd_svc_stats
which now requires other concurrent access protection than
simply to remove external tunables in /sys or /debug.

Fixes: 54a64ea818 ("LU-18446 ptlrpc: lower CPUs latency during client I/O")
Signed-off-by: Bruno Faccini <bfaccini@nvidia.com>
Change-Id: I45a5f65216fa2bf0821776ff3141fa8e2a33f10e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59593
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>