EX-8236 pcc: abort in-progress attach by PCC detach command

EX-8236 pcc: abort in-progress attach by PCC detach command

A user may want to abort in-progress attach for some purposes such
as freeing space for PCC backend.
To support this operation, we add an "abort" option for PCC detach
command to abort the in-progress attach.

Change-Id: I49fb1c42838f8d7e9728a5c4c6f3d60e959b233b
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52375
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>