rocker: fix memory leaks of fib_work on two error return paths

Currently there are two error return paths that leak memory allocated
to fib_work. Fix this by kfree'ing fib_work before returning.

Addresses-Coverity: ("Resource leak")
Fixes: 19a9d136 ("ipv4: Flag fib_info with a fib_nh using IPv6 gateway")
Fixes: dbcc4fa7 ("rocker: Fail attempts to use routes with nexthop objects")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>