classifier: Use rculist.

The list of identical, but lower priority rules is not currently used
in classifier lookup.  A later patch introducing conjunctive matches
needs to access the list during lookups, so we must make the list RCU.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>