LU-15217 pcc: disable PCC for encrypted files

LU-15217 pcc: disable PCC for encrypted files

When files are encrypted in Lustre using fscrypt, they should
normally not be accessible to users without the proper encyrption
key. However, if a user has then encryption key loaded when they
read a file, it may be decrypted in memory and saved to the PCC
backend in unencrypted form.

Due to the above reason, we just disable PCC caching for encrypted
files.

DDN-bug-id: EX-3571
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I6c363dcad7a6bc8520350c0295f6e221bec3abb0
Reviewed-on: https://review.whamcloud.com/45545
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>