ospf6d: keep track of the socket set thread

ospf6d: keep track of the socket set thread

When using the timer to set the socket multicast options, keep track
of the thread pointer. If we lose the thread reference we might have
situations where multicast is enabled when it should be disabled and
vice versa.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>