dpif: Add 'used' argument to dpif_flow_stats_extract().

dpif: Add 'used' argument to dpif_flow_stats_extract().

The following commit will need to use a value other than a literal
time_msec() in one case.  This commit is just preparation.

Factoring the time_msec() call out of the loop in
handle_flow_miss_without_facet() is a really minor optimization.  It isn't
the main point here.

Signed-off-by: Ben Pfaff <blp@nicira.com>