dpif-netdev: Update comment about flow installation race.

Userspace datapath uses per-PMD flow tables/classifiers for a long
time. However, it was decided to keep this race window to not block
revalidators. Comment should be updated to reflect the current state.

Fixes: 1c1e46e ("dpif-netdev: Add per-pmd flow-table/classifier.")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>

lib/dpif-netdev.c

@@ -5126,8 +5126,7 @@ handle_packet_upcall(struct dp_netdev_pmd_thread *pmd,
          * could have already been installed since we last did the flow
          * lookup before upcall.  This could be solved by moving the
          * mutex lock outside the loop, but that's an awful long time
-         * to be locking everyone out of making flow installs.  If we
-         * move to a per-core classifier, it would be reasonable. */
+         * to be locking revalidators out of making flow modifications. */
         ovs_mutex_lock(&pmd->flow_mutex);
         netdev_flow = dp_netdev_pmd_lookup_flow(pmd, key, NULL);
         if (OVS_LIKELY(!netdev_flow)) {