cfm: update remote opstate only when a CCM is received.

cfm: update remote opstate only when a CCM is received.

The remote opstate for a CFM interface is presumed to be up unless a CCM is
received which signals opstate down. This means than an interface configured
for CFM demand mode may incorrectly appear to be opstate up if it has not
received a CCM within the last fault interval.

We should remember the last remote opstate for a CFM interface and only
change it when a CCM arrives signaling a change.

Bug #18806
Signed-off-by: Paul Ingram <pingram@nicira.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>