ofproto: Maintain "expirable" list for "add" flow_mod with replaced rule.

Commit e503cc199 (ofproto: Optimise OpenFlow flow expiry) optimized
OpenFlow flow expiration by putting expirable flows on a list, but it
failed to remove flows from the list when they were replaced by a new
flow with an OpenFlow flow_mod "add" operation.  This commit fixes the
problem.

Found by valgrind.

CC: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>