ip6mr: only set ip6mr_table from setsockopt when ip6mr_new_table succeeds

Currently, raw6_sk(sk)->ip6mr_table is set unconditionally during
ip6_mroute_setsockopt(MRT6_TABLE). A subsequent attempt at the same
setsockopt will fail with -ENOENT, since we haven't actually created
that table.

A similar fix for ipv4 was included in commit 5e1859fb ("ipv4: ipmr:
various fixes and cleanups").

Fixes: d1db275d ("ipv6: ip6mr: support multiple tables")
Signed-off-by: NSabrina Dubroca <sd@queasysnail.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6mr.c

@@ -1760,7 +1760,8 @@ int ip6_mroute_setsockopt(struct sock *sk, int optname, char __user *optval, uns
 		ret = 0;
 		if (!ip6mr_new_table(net, v))
 			ret = -ENOMEM;
-		raw6_sk(sk)->ip6mr_table = v;
+		else
+			raw6_sk(sk)->ip6mr_table = v;
 		rtnl_unlock();
 		return ret;
 	}