netdev-linux: Use underlying tap device on netdev_linux_listen().

netdev-linux: Use underlying tap device on netdev_linux_listen().

Commit acf608 (ofproto-dpif: Use a single underlying datapath across
multiple bridges.) broke connectivity to userspace datapath devices.
The code assumed the first caller to open a tap device with
netdev_linux_open() wanted to write to it.  This commit moves that logic
to when netdev_linux_listen() is called.

Thanks to Ben Pfaff for helping debug the issue.

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Reported-by: Simon Horman <horms@verge.net.au>
Tested-by: Simon Horman <horms@verge.net.au>