vxlan: 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, reverse destination and source
addresses, so that Open vSwitch is able to match this packet against
the existing, reverse flow.

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>