ipv4: Check attribute length for RTA_FLOW in multipath route

Make sure RTA_FLOW is at least 4B before using.

Fixes: 4e902c57

 ("[IPv4]: FIB configuration using struct fib_config")
Signed-off-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>