ipv6: check sk sk_type and protocol early in ip_mroute_set/getsockopt

Commit 5e1859fb ("ipv4: ipmr: various fixes and cleanups") fixed
the issue for ipv4 ipmr:

  ip_mroute_setsockopt() & ip_mroute_getsockopt() should not
  access/set raw_sk(sk)->ipmr_table before making sure the socket
  is a raw socket, and protocol is IGMP

The same fix should be done for ipv6 ipmr as well.

This patch can fix the panic caused by overwriting the same offset
as ipmr_table as in raw_sk(sk) when accessing other type's socket
by ip_mroute_setsockopt().
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6mr.c

@@ -1666,6 +1666,10 @@ int ip6_mroute_setsockopt(struct sock *sk, int optname, char __user *optval, uns
 	struct net *net = sock_net(sk);
 	struct mr6_table *mrt;
 
+	if (sk->sk_type != SOCK_RAW ||
+	    inet_sk(sk)->inet_num != IPPROTO_ICMPV6)
+		return -EOPNOTSUPP;
+
 	mrt = ip6mr_get_table(net, raw6_sk(sk)->ip6mr_table ? : RT6_TABLE_DFLT);
 	if (!mrt)
 		return -ENOENT;
@@ -1677,9 +1681,6 @@ int ip6_mroute_setsockopt(struct sock *sk, int optname, char __user *optval, uns
 
 	switch (optname) {
 	case MRT6_INIT:
-		if (sk->sk_type != SOCK_RAW ||
-		    inet_sk(sk)->inet_num != IPPROTO_ICMPV6)
-			return -EOPNOTSUPP;
 		if (optlen < sizeof(int))
 			return -EINVAL;
 
@@ -1815,6 +1816,10 @@ int ip6_mroute_getsockopt(struct sock *sk, int optname, char __user *optval,
 	struct net *net = sock_net(sk);
 	struct mr6_table *mrt;
 
+	if (sk->sk_type != SOCK_RAW ||
+	    inet_sk(sk)->inet_num != IPPROTO_ICMPV6)
+		return -EOPNOTSUPP;
+
 	mrt = ip6mr_get_table(net, raw6_sk(sk)->ip6mr_table ? : RT6_TABLE_DFLT);
 	if (!mrt)
 		return -ENOENT;