ovs-ofctl: Fix "cookie mask" issue in diff-flows command.

ovs-ofctl: Fix "cookie mask" issue in diff-flows command.

Commit e729e79 (Add ability to restrict flow mods and flow stats
requests to cookies.) introduced a cookie_mask field to the
ofputil_flow_stats_request struct that allowed restricting the flows to
a particular cookie pattern.  The diff-flows command uses the
read_flows_from_switch() function, which did not properly initialize
this field, which would cause it to miss flows.  This commit sets the
value to zero, which allows any cookie.

Bug #8984

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Reported-by: Luca Giraudo <lgiraudo@nicira.com>