LU-17249 ptlrpc: protect scp_rqbd_idle list operations

LU-17249 ptlrpc: protect scp_rqbd_idle list operations

Protect scp_rqbd_idle list entry getting by spinlock
in ptlrpc_service_purge_all() like it does in all
other places where rqbd_list linkage is being managed

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Iace37b1ee79bfd0c3a54a35722952e17d860a91c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52931
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>