EX-3133 pcc: keep PCC copy when it is being attached

EX-3133 pcc: keep PCC copy when it is being attached

When detach a file from PCC backend via FID, if the file is being
attached, it should not purge the coresponding PCC copy from the
PCC backend. Just keep the PCC copy to finish the attach process.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I8a8f7c6986d51eaf9b2516e5dd5a6f21aa38b7db
Reviewed-on: https://review.whamcloud.com/43637
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>