isisd: link protection optional fallback in ti-lfa
The current implementation of TI-LFA computes link-protecting
repair paths (even when node protection is enabled) to have repair
paths to all destinations when no node-protecting repair has been
found. This may be desired or not. E.g. the link-protecting paths
may use the protected node and be, therefore, useless if the node
fails. Also, computing link-protecting repairs incurs extra
calculations.
With this patch, when node protection is enabled, link protecting
repair paths are only computed if "link-fallback" is specified in
the configuration, on a per interface and IS-IS level.