isisd: fix interface ldp-sync configuration

isisd: fix interface ldp-sync configuration

There are two checks done when configuring ldp-sync on an interface:
- interface is not a loopback
- interface is in the default VRF
Both checks are incorrectly done using the operational data.

The second check can be done using only config data - do that.

The first check can't be done using only configurational data, but it's
not necessary. LDP sync code doesn't operate on loopback interfaces
already. There's no harm in allowing this to be configured.

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