isisd: detect Prefix-SID collisions and handle them appropriately

isisd: detect Prefix-SID collisions and handle them appropriately

isisd relies on its YANG module to prevent the same SID index
from being configured multiple times for different prefixes. It's
possible, however, to have different routers assigning the same SID
index for different prefixes. When that happens, we say we have a
Prefix-SID collision, which is ultimately a misconfiguration issue.

The problem with Prefix-SID collisions is that the Prefix-SID that
is processed later overwrites the previous ones. Then, once the
Prefix-SID collision is fixed in the configuration, the overwritten
Prefix-SID isn't reinstalled since it's already marked as installed
and it didn't change. To prevent such inconsistency from happening,
add a safeguard in the SPF code to detect Prefix-SID collisions and
handle them appropriately (i.e. log a warning + ignore the Prefix-SID
Sub-TLV since it's already in use by another prefix). That way,
once the configuration is fixed, no Prefix-SID label entry will be
missing in the LFIB.

Reported-by: Emanuele Di Pascale <emanuele@voltanet.io>
Signed-off-by: Renato Westphal <renato@opensourcerouting.org>