ofproto-dpif: Set need_revalidate when removing cfm from ofport.

ofproto-dpif: Set need_revalidate when removing cfm from ofport.

When cfm is deleted from a port, all modules should release their
reference so that the cfm struct can be removed from the global hmap
and freed.  Therein, the reference held by xlate module can only be
released when the need_revalidate flag is set (e.g set to
REV_RECONFIGURE).  And this flag should be set while removing cfm
from ofport.  Unfortunately, this has never been done before and the
bug was hidden by another bug fixed in recent commit a190839
(netdev-vport: Do not update netdev when there is no config change.)

To fix this issue, this commit makes the code set need_revalidate
when removing cfm from ofport.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>