ospfd: explicitly exit from the router node

ospfd: explicitly exit from the router node

There's a new "mpls ldp-sync" command added to the OSPF router node in
FRR 8.0. This change broke the following config:
```
router ospf
!
mpls ldp
 discovery hello interval 10
!
```
The config was broken because the "mpls ldp" line is now treated as an
"mpls ldp-sync" line inside the router node. We must explicitly print
"exit" at the end of OSPF router node to fix the issue.

Fixes #9206.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>