isisd: consult locally configured MSD when computing TI-LFA repair paths

isisd: consult locally configured MSD when computing TI-LFA repair paths

Do not attempt to install a TI-LFA backup nexthop if its number of
labels exceeds the locally configured MSD (Maximum Stack Depth). The
idea is to prevent forward-plane installation failures before they
happen. The MSD check should also allow the "show isis fast-reroute
summary" command (not implemented yet) to display the actual
protection coverage provided by TI-LFA, which might not be 100%
if the MSD isn't big enough.

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