patch:
- Do the correct test for ldt allocation
- Drop dt_lock just before to call kmem_free (since it acquires blocking
locks inside)
- Solve a deadlock with smp_rendezvous() where other CPU will wait
undefinitively for dt_lock acquisition.
- Add dt_lock in the WITNESS list of spinlocks
While applying these modifies, change the requirement for user_ldt_free()
making that returning without dt_lock held.
Tested by: marcus, tegge
Reviewed by: tegge
Approved by: jeff (mentor)
02b0a160dc