cfm: Define old_cfm_fault as 'enum cfm_fault_reason'.

CFM fault variable type has been changed to 'enum cfm_fault_reason' for
long time.  However, inside cfm_run(), the old_cfm_fault is still defined
as boolean.  This commit fixes the issue.

Found by inspection.

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