LU-14677 sec: do not expose security.c to listxattr/getxattr

LU-14677 sec: do not expose security.c to listxattr/getxattr

security.c xattr, which contains encryption context, should not be
exposed by the xattr-related system calls such as listxattr() and
getxattr() because of its special semantics.
Update sanity-sec test_57 to test this.

Lustre-change: https://review.whamcloud.com/44101
Lustre-commit: TBD (db49ec2ae2c96a09fae054e5fcb3d1959e26f83d)

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I919f5cbafc53f5745fbfb5b9d2d7316e892d8c9f
Reviewed-on: https://review.whamcloud.com/44183
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>