zebra: enqueue NHG_DEL in rib_nhg meta queue

zebra: enqueue NHG_DEL in rib_nhg meta queue

The NHG_DEL operation is done directly from ZAPI call, whereas
the NHG_ADD operation is done in the rib_nhg meta queue.

This may be problematic when ADD is followed by DEL. Imagine a
scenarion with two protocol NHIDs. <NH1> depends of <NH2> and
<NH3>. The deletion of <NH3> at the protocol level will trigger
2 messages to ZEBRA: NHG_ADD(<NH1>) and NHG_DEL(<NH3>).

Those operations are properly enqueued in ZAPI, but in the end,
the NHG_DEL is executed first. This causes NHG_ADD to unlink an
already freed NHG.

Fix this by consistently enqueuing NHG_DEL and NHG_ADD operations.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>