classifier: Make traversing identical rules robust.

classifier: Make traversing identical rules robust.

The traversal of the list of identical rules from the lookup threads
is fragile if the list head is removed during the list traversal.

This patch simplifies the implementation of that list by making the
list NULL terminated, singly linked RCU-protected list.  By having the
NULL at the end there is no longer a possiblity of missing the point
when the list wraps around.  This is significant when there can be
multiple elements with the same priority in the list.

This change also decreases the size of the struct cls_match back
pre-'visibility' attribute size.

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