dpif-netdev: Move decoding of data out of dp_netdev_output_userspace()

This is in preparation for making dp_netdev_action_userspace()
more generic and passing dp_netdev_output_userspace() as a callback.
In this case it makes sense to decode userdata in generic code.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>