dpif-netdev: Fix rare flow add race condition.

dpif-netdev: Fix rare flow add race condition.

Before this patch, dp_netdev_flow_add() inserted newly minted flows in
the "flow_table" cmap before inserting them into the per core "dpcls"
classifier.  Since dpcls_insert() initializes 'flow->cr.mask', there's
a brief window where the flow is accessible from the cmap, but has a
bogus mask value.

In my testing, under rare instances (i.e. once every 20 minutes with a
very specific flow table and traffic pattern), revalidators core dump
when they call dpif_netdev_flow_dump_next(), which accesses this bogus
mask value from dp_netdev_flow_to_dpif_flow().

By inserting into the per core classifier before the cmap, all the
values are guaranteed to be initialized during flow dumps.  With this
patch, I can no longer reproduce the crash.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>