selinux: initialize proto variable in selinux_ip_postroute_compat()

Clang static analysis reports this warning

hooks.c:5765:6: warning: 4th function call argument is an uninitialized
                value
        if (selinux_xfrm_postroute_last(sksec->sid, skb, &ad, proto))
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

selinux_parse_skb() can return ok without setting proto.  The later call
to selinux_xfrm_postroute_last() does an early check of proto and can
return ok if the garbage proto value matches.  So initialize proto.

Cc: stable@vger.kernel.org
Fixes: eef9b41 ("selinux: cleanup selinux_xfrm_sock_rcv_skb() and selinux_xfrm_postroute_last()")
Signed-off-by: Tom Rix <trix@redhat.com>
[PM: typo/spelling and checkpatch.pl description fixes]
Signed-off-by: Paul Moore <paul@paul-moore.com>

Author: Tom Rix

security/selinux/hooks.c

@@ -5785,7 +5785,7 @@ static unsigned int selinux_ip_postroute_compat(struct sk_buff *skb,
 	struct sk_security_struct *sksec;
 	struct common_audit_data ad;
 	struct lsm_network_audit net = {0,};
-	u8 proto;
+	u8 proto = 0;
 
 	sk = skb_to_full_sk(skb);
 	if (sk == NULL)