isisd: update Node-SID flag dynamically

isisd: update Node-SID flag dynamically

Node-SIDs refer to Prefix-SIDs associated with host prefixes of
loopback addresses. As such, whenever an interface address is added
or deleted, all configured Prefix-SIDs must be reevaluated to check
if the N-flag needs to be set or unset.

This change fixes some race conditions in the TI-LFA topotest where
specific sequence of events could cause Prefix-SIDs to not have the
N-flag set when they should, resulting in various failures.

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