dpif-netdev: Update comment about flow installation race.

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: 1c1e46ed8457 ("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>