LU-12373 pcc: uncache the pcc copies when remove a PCC backend

LU-12373 pcc: uncache the pcc copies when remove a PCC backend

Currently when remove a PCC backend from a client, it does not
make any special handling for previously cached files at all.
Users can still use PCC caching service for these files. This
may not what users want. The reason is as follows:

1) For RW-PCC cached files, it does not restore the data back
into Lustre OSTs of the main filesystem. Although the PCC
backend falls back as a tranditional HSM storage solution
since the lhsmtool_posix copytool is still running at this
client. But this is dangerous, and likly to cause user data
to be lost if the PCC device may be permanently unavailable.

2) The space used by these PCC cached files may not released.

In this patch, when remove a PCC backend from a client, the
default action is to scan the PCC backend fs, uncache
(detach and remove) the PCC copy from PCC by FID.

We also add an option "--keep|-k" for PCC backend removal.
It behaves as before, just remove the PCC backend, but
retain the data on the cache.

This patch also introduces a common library to scan the HSM
backend.

Test-Parameters: clientcount=3 testlist=sanity-pcc,sanity-pcc,sanity-pcc
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Ib4db36137c025fd78c7022c8b8c39b63e3b9ad4d
Reviewed-on: https://review.whamcloud.com/41919
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>