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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31470