datapath: Convert ODP_VPORT_* to use AF_NETLINK socket layer.

This commit calls genl_lock() and thus doesn't support Linux before
2.6.35, which wasn't exported before that version.  That problem will
be fixed once the whole userspace interface transitions to Generic
Netlink a few commits from now.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>