ofproto-dpif: Probe for userdata after backer is fully operational.

When probing for variable length userdata before handler threads are
set, the pid included in the userspace action will be 0, which is
flagged as an error by the linux kernel datapath.  As a result the
feature probe will produce an unnecessary log message.  By probing for
variable length userdata later the probe works as intended and the
unnecessary log message is avoided.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>