datapath: An expanded table should be larger than its predecessor

datapath: An expanded table should be larger than its predecessor

This resolves what appears to be a think-o in tbl_expand()

* Old Logic: Always create tables with TBL_MIN_BUCKETS buckets
* New Logic: Create tables twice as big as their predecessor

When sending 10,000 flows through ovs-vswitchd:
* Old Logic: CPU bound in tbl_lookup(), significant packet loss
* New Logic: ~10% of one core used, negligible packet loss

Tested with an Intel E5520 @ 2.27GHz,
flows from an ethernet device to to a dummy interface with
no address configured.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Jesse Gross <jesse@nicira.com>