kernel - Fix serious issue w/ smp_invltlb(), plus other issues (3)

kernel - Fix serious issue w/ smp_invltlb(), plus other issues (3)

* splz() and doreti on both i386 and x86_64 were leaving interrupts
  disabled through calls to e.g. sched_ithd() and IPI function callbacks.

  As before this can lead to a deadlock if these functions call
  smp_invltlb() or some other cpu calls smp_invltlb() while these
  functions are spinning on a full IPIQ.

* sti in appropriate places to fix the problem.