ipv6: fix return value check in fib6_add()

In case of error, the function fib6_add_1() returns ERR_PTR()
or NULL pointer. The ERR_PTR() case check is missing in fib6_add().

dpatch engine is used to generated this patch.
(https://github.com/weiyj/dpatch)
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -819,6 +819,10 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt, struct nl_info *info)
 					offsetof(struct rt6_info, rt6i_src),
 					allow_create, replace_required);
 
+			if (IS_ERR(sn)) {
+				err = PTR_ERR(sn);
+				sn = NULL;
+			}
 			if (!sn) {
 				/* If it is failed, discard just allocated
 				   root, and then (in st_failure) stale node