udpif: Bug fix updif_flush

Found during development.

Before this commit, all datapath flows are cleared with dpif_flush(),
but the revalidator thread still holds ukeys, which are caches of the
datapath flows in the revalidaor.  Flushing ukeys causes flow_del
messages to be sent to the datapath again on flows that have been
deleted by the dpif_flush() already.

Double deletion by itself is not problem, per se, may an efficiency
issue. However, for ever flow_del message sent to the datapath, a log
message, at the warning level, will be generated in case datapath
failed to execute the command. In addition to cause spurious log
messages, Double deletion causes unit tests to report erroneous
failures as all warning messages are considered test failures.

The fix is to simply shut down the revalidator threads to flush all
ukeys, then flush the datapth before restarting the revalidator threads.

dpif_flush() was implemented as flush flows of all datapaths while
most of its invocation should only flush its local datapath.
Only megaflow on/off commands should flush all dapapaths. This bug is
also fixed.

Change-Id: I2312993f818a7f98c8690d9520507c326580b88c
Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
Reviewed-by: Ciara Loftus <ciara.loftus@intel.com>
Reviewed-by: Stephen Finucane <stephen.finucane@intel.com>