EX-8027 pcc: wait for in-progress attaches when remove PCC

EX-8027 pcc: wait for in-progress attaches when remove PCC

When remove a PCC backend from a client, it should wait for all
in-progress attaches finished. Othwise, it results in the failure
of the PCC backend umount operation.

The reason is that the PCC copy is referenced in the kernel, not
used by any applications in user space and the tool "lsof" can not
check whether the target PCC backend is used or not.

Change-Id: I05b268e75841f9f17e77819ed20c85c78d7c6ad6
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51940
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Feng Lei <flei@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>