ldpd: fix missing label mappings

In some scenarios, it's possible to send a Label Withdraw to a neighbor
and not receive a corresponding Label Release right away. This can happen
during reconvergence after a network failure or configuration change.

When this happens, the list of upstream mappings of a given FEC might
not be empty even after sending a Label Withdraw to all neighbors. This
situation holds until all neighbors either send a Label Release or are
torn down (e.g. keepalive timeout).

With that said, we shouldn't check for 'RB_EMPTY(&fn->upstream)'
in lde_kernel_update() because it can prevent ldpd from sending label
mappings in such circumstances. This check was introduced to avoid sending
the same label mapping more than once to the same neighbor, but we need
to remove this optimization for now until we find a better solution (which
probably involves refactoring the whole zebra<->ldpd communication).

While here, add a new debug message in lde_send_labelmapping() which
can aid in troubleshooting label problems in the future.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

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