classifier: Make insert and replace take a const rule, too.

classifier: Make insert and replace take a const rule, too.

classifier_remove() was recently changed to take a const struct
cls_rule *.  Make the corresponding change to classifier_replace() and
classifier_insert().  This simplifies existing calling sites in
ofproto.

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