bfd: Fix cpath_down set failure.

Commit ccc09689 (bfd: Implement Bidirectional Forwarding Detection.)
set the bfd local diagnostic to "Concatenated Path Down" in response
to the set of cpath_down only when the current local diagnostic is
"None".  However, since the bfd local diagnostic is not reset when
the bfd state is restored from last erroneous state, the set of
cpath_down will not update the local diagnostic in that case.

This commit fixes the bug by always checking for local diagnostic
change when cpath_down is set or reset.

Bug #22625
Signed-off-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>