geneve: Support for PMTU discovery on directly bridged links

If the interface is a bridge or Open vSwitch port, and we can't
forward a packet because it exceeds the local PMTU estimate,
trigger an ICMP or ICMPv6 reply to the sender, using the same
interface to forward it back.

If metadata collection is enabled, set destination and source
addresses for the flow as if we were receiving the packet, so that
Open vSwitch can match the ICMP error against the existing
association.

v2: Use netif_is_any_bridge_port() (David Ahern)

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>