datapath: Fix multipart datapath dumps.

The logic to split up the list of datapaths into multiple Netlink messages
was simply wrong, causing the list to be terminated after the first part.
Only about the first 50 datapaths would be dumped.  This fixes the
problem.

Bug #9124.
Reported-by: Paul Ingram <paul@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>