vxlan: listen on multiple ports

The commit 823aa873
  Author: stephen hemminger <stephen@networkplumber.org>
  Date:   Sat Apr 27 11:31:57 2013 +0000

    vxlan: allow choosing destination port per vxlan

introduced per-vxlan UDP port configuration but only did half of the
necessary work.  It added per vxlan destination for sending, but
overlooked the handling of multiple ports for incoming traffic.

This patch changes the listening port management to handle multiple
incoming UDP ports. The earlier per-namespace structure is now a hash
list per namespace.

It is also now possible to define the same virtual network id
but with different UDP port values which can be useful for migration.
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>