net: gain ipv4 mtu when mtu is not locked

When mtu is locked, we should not obtain ipv4 mtu as we return immediately
in this case and leave acquired ipv4 mtu unused.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>