nd6: Mark several callouts as MPSAFE

The use of Giant here is vestigal and does not provide any useful
synchronization. Furthermore, non-MPSAFE callouts can cause the
softclock threads to block waiting for long-running newbus operations to
complete.

Reported by: mav
Reviewed by: bz
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 663428ea17e3a81f4c514d2571b90a13c065b1e8)