security: fix description of values returned by cap_inode_need_killpriv

cap_inode_need_killpriv returns 1 if security.capability exists and
has a value and inode_killpriv() is required, 0 otherwise. Fix the
description of the return value to reflect this.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>