ldpd: fix wrong gtsm count

ldpd: fix wrong gtsm count

In linux networking stack, the received mpls packets will be processed
by the host *twice*, one as mpls packet, the other as ip packet, so
its ttl decreased 1.

So, we need release the `IP_MINTTL` value if gtsm is enabled, it is for the
mpls packets of neighbor session caused by the command:
`label local advertise explicit-null`.

This change makes the gtsm mechanism a bit deviation.

Fix PR #8313

Signed-off-by: anlan_cs <vic.lan@pica8.com>
(cherry picked from commit 1919df3a64d3fe6d4084c1d0b050b3e368860170)