capabitlies: ns_capable can use the cap helpers rather than lsm call

Just to reduce the number of places to change if we every change the LSM
hook, use the capability helpers internally when possible.
Signed-off-by: NEric Paris <eparis@redhat.com>
Acked-by: NSerge E. Hallyn <serge.hallyn@canonical.com>

kernel/capability.c

@@ -384,7 +384,7 @@ bool ns_capable(struct user_namespace *ns, int cap)
 		BUG();
 	}
 
-	if (security_capable(current_cred(), ns, cap) == 0) {
+	if (has_ns_capability(current, ns, cap)) {
 		current->flags |= PF_SUPERPRIV;
 		return true;
 	}