LU-14989 sec: keep encryption context in xattr cache

LU-14989 sec: keep encryption context in xattr cache

When an inode is being cleared, its xattr cache must be completely
wiped. But in case of lock cancel, we want to keep the encryption
context, as further processing might need to check it.

Fixes: 1faf54e8bf ("LU-14989 sec: access to enc file's xattrs")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I8a2f4497129353a7fbf86cdaaa13fae6e0988790
Reviewed-on: https://review.whamcloud.com/45148
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>