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>
(cherry picked from commit f80003b0e12a090b34e2ab9547ed072ff02c33ba)