LU-10092 pcc: change detach behavior and add keep option

LU-10092 pcc: change detach behavior and add keep option

After introduce the feature of auto-attach at open, when the PCC
cached file is detach by "pcc detach" command, it will be attached
automatically at the next open. This may be not what the user wants.

To solve this problem, we change the defualt detach behavior and
add an option "--keep|-k" for the detach of RW-PCC.
The manual "lfs pcc detach" command will detach the file from PCC
permanently. And it will also remove the PCC copy by default.
When the file is detached with "keep" option, it only unmaps the
relationship between the file inode and PCC copy, but keep the
PCC copy. The file is allowed to be attached automatically at
the next open when the file is still valid in cache.

Note here that currently auto detach caused by inode reclaim or
revocation of the layout lock would not delete the PCC copy too.

Test-Parameters: clientcount=3 testlist=sanity-pcc,sanity-pcc,sanity-pcc
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I010df54177ae4cfeddcc0a9982c1aee58ee683de
Reviewed-on: https://review.whamcloud.com/33844
Tested-by: Jenkins
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>