ipv6: Do cleanup if attribute validation fails in multipath route

As Nicolas noted, if gateway validation fails walking the multipath
attribute the code should jump to the cleanup to free previously
allocated memory.

Fixes: 1ff15a71 ("ipv6: Check attribute length for RTA_GATEWAY when deleting multipath route")
Signed-off-by: NDavid Ahern <dsahern@kernel.org>
Acked-by: NNicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20220103170555.94638-1-dsahern@kernel.orgSigned-off-by: NJakub Kicinski <kuba@kernel.org>

net/ipv6/route.c

@@ -5277,12 +5277,10 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
 
 			nla = nla_find(attrs, attrlen, RTA_GATEWAY);
 			if (nla) {
-				int ret;
-
-				ret = fib6_gw_from_attr(&r_cfg.fc_gateway, nla,
+				err = fib6_gw_from_attr(&r_cfg.fc_gateway, nla,
 							extack);
-				if (ret)
-					return ret;
+				if (err)
+					goto cleanup;
 
 				r_cfg.fc_flags |= RTF_GATEWAY;
 			}