EX-5427 sec: fix pcc detach of encrypted file

EX-5427 sec: fix pcc detach of encrypted file

In case of 'lfs pcc detach' of an encrypted file, its page cache
pages must be trashed as they might contain cipher text because
of the pcc file being detached.

Improve sanity-pcc test_21j to exercise this use case.

Fixes: 393697cac0 ("EX-4182 sec: support of PCC-RO for encrypted files")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ice13c5b4205c074d9b46e2175d18f8743dbe9c58
Reviewed-on: https://review.whamcloud.com/47766
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>