ipv4: Don't create nh exeption when the device mtu is smaller than the reported pmtu

When a local tool like tracepath tries to send packets bigger than
the device mtu, we create a nh exeption and set the pmtu to device
mtu. The device mtu does not expire, so check if the device mtu is
smaller than the reported pmtu and don't crerate a nh exeption in
that case.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>