EX-8236 pcc: abort data copy when clear PCC backend

EX-8236 pcc: abort data copy when clear PCC backend

This patch adds an option "--abort" for "lctl pcc del|clear"
command tools.
With this option, the user will first set ATTACH_ABORTING flag on
all in-progress attaching files, and then wait for them to abort
the attache when remove a PCC backend from a client.

Add sanity-pcc/test_108 to verify it.

Change-Id: I4e2f3ec8866e9af45f4524a9f45ee418ef4cb5be
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53373
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>