When an inode doesn't support xattrs, turn listxattr off as well.

(When xattrs are "turned off", the VFS still passes security xattr
operations through to security modules, which can still expose inode
security labels that way.)
Signed-off-by: NAndreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>