geneve, vxlan: Don't set exceptions if skb->len < mtu

geneve, vxlan: Don't set exceptions if skb->len < mtu

We shouldn't abuse exceptions: if the destination MTU is already higher
than what we're transmitting, no exception should be created.

Fixes: 483aabf41299 ("geneve: update skb dst pmtu on tx path")
Fixes: 7be0638ed5db ("vxlan: update skb dst pmtu on tx path")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>