lib/classifier: Minimize critical section.

classifier_find_rule_exactly() only needs the mutex to protect the
list traversal.  Subtables are already RCU protected.

Locking here can be eliminated completely when RCU list is available.

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