[MLSXFRM]: Fix build with SECURITY_NETWORK_XFRM disabled.

The following patch will fix the build problem (encountered by Andrew
Morton) when SECURITY_NETWORK_XFRM is not enabled.

As compared to git-net-selinux_xfrm_decode_session-build-fix.patch in
-mm, this patch sets the return parameter sid to SECSID_NULL in
selinux_xfrm_decode_session() and handles this value in the caller
selinux_inet_conn_request() appropriately.

Signed-off-by: Venkat Yekkirala <vyekkirala@TrustedCS.com>
Acked-by: James Morris <jmorris@namei.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 1dc935f7..33028b3 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3591,6 +3591,11 @@
 	err = selinux_xfrm_decode_session(skb, &peersid, 0);
 	BUG_ON(err);
 
+	if (peersid == SECSID_NULL) {
+		req->secid = sksec->sid;
+		return 0;
+	}
+
 	err = security_sid_mls_copy(sksec->sid, peersid, &newsid);
 	if (err)
 		return err;