fib: Don't back walk from a path-list when a child fist attaches.

Type: fix

If the walk is triggered when the child is added, then that child is visited in the walk. However, since it is just attahcing to the path-list it may not, or indeed cannot, have all the context it needs to successfully handle the walk.
In the case of MPLS tunnel, it does not have the path extensions ready, and cannot since the path extensions need to resolve on the path-list.

Signed-off-by: Neale Ranns <neale@graphiant.com>
Change-Id: I027af8cf2f522d2f6e37931bea60c767f0cb667d