isisd: guard against adj timer display overflow

isisd: guard against adj timer display overflow

An adjacency should be removed when the holdtimer expires, but if the
system is overloaded we may end up doing it late. In the meanwhile vtysh
will display an incorrect value in the show isis neighbor output, due to
an overflow of the unsigned variable used to display the Holdtime, e.g.:

pe1# show isis neighbor
Area test:
 System Id     Interface   L   state   Holdtime  SNPA
 Spirent-1     2.201       1   Down    26        2020.2020.2020
 Spirent-1     2.203       1   Up      21        2020.2020.2020
 Spirent-1     2.204       1   Up      18446744073709551615  2020.2020.2020
 Spirent-1     2.207       1   Up      18446744073709551615  2020.2020.2020
 Spirent-1     2.208       1   Up      18446744073709551615  2020.2020.2020
 Spirent-1     2.209       1   Up      0         2020.2020.2020
 Spirent-1     2.210       1   Up      18446744073709551615  2020.2020.2020
 pe2           12.200      1   Up      30        2020.2020.2020

Guard against that by printing an "Expiring" message instead.

Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>