datapath/flow_netlink: Avoid wildcarding tunnel key with disabled megaflows

If the userspace wants to match on a flow with some tunnel attributesset to 0,
it simply omits them in the netlink attributes stream.
Since our wildcarding logic (when megaflows are disabled) is based on the
attributes in the netlink stream, we set our mask incorrectly.

This commit adds a check to detect if the userspace wants to match on a tunnel,
in which case we simply unwildcard the whole tun_key

Reported-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>