ip_gre: get key from session_id correctly in erspan_rcv

erspan only uses the first 10 bits of session_id as the key to look
up the tunnel. But in erspan_rcv, it missed 'session_id & ID_MASK'
when getting the key from session_id.

If any other flag is also set in session_id in a packet, it would
fail to find the tunnel due to incorrect key in erspan_rcv.

This patch is to add 'session_id & ID_MASK' there and also remove
the unnecessary variable session_id.

Fixes: 84e54fe ("gre: introduce native tunnel support for ERSPAN")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -259,7 +259,6 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 	struct ip_tunnel *tunnel;
 	struct erspanhdr *ershdr;
 	const struct iphdr *iph;
-	__be32 session_id;
 	__be32 index;
 	int len;
 
@@ -275,8 +274,7 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 	/* The original GRE header does not have key field,
 	 * Use ERSPAN 10-bit session ID as key.
 	 */
-	session_id = cpu_to_be32(ntohs(ershdr->session_id));
-	tpi->key = session_id;
+	tpi->key = cpu_to_be32(ntohs(ershdr->session_id) & ID_MASK);
 	index = ershdr->md.index;
 	tunnel = ip_tunnel_lookup(itn, skb->dev->ifindex,
 				  tpi->flags | TUNNEL_KEY,