net/netlink: fix memory leak in netlink_get_neighbor

Add kmm_free(alloc) before returning NULL when neigh is NULL in
netlink_get_neighbor(). Without this, the allocated memory is
leaked on the error path.

Signed-off-by: zhanghongyu <zhanghongyu@xiaomi.com>

net/netlink/netlink_route.c

@@ -694,6 +694,7 @@ netlink_get_neighbor(FAR const void *neigh, int domain, int type,
     {
       if (neigh == NULL)
         {
+          kmm_free(alloc);
           return NULL;
         }