Change the mb() use in the sched_ult tdq_notify() and sched_idletd()
to more C11-ish atomic_thread_fence_seq_cst().

Note that on PowerPC, which currently uses lwsync for mb(), the change
actually fixes the missed store/load barrier, intended by r271604 [*].

Reviewed by: alc
Noted by: alc [*]
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks