EX-8027 pcc: add wait option when remove a PCC backend

EX-8027 pcc: add wait option when remove a PCC backend

In this patch, we add a "wait" option for the PCC tool when remove
PCC backend from a client:
	lctl pcc del --wait $MOUNT $pcc_path
	lctl pcc clear --wait $MOUNT

With this option, the caller must wait for all in-progress
attaches finished when remove the PCC backend from a client.

Change-Id: Ic8386329087a7129b0583fa823cbb50673893d0d
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51944
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>