the same vmspace, decrement the reference count of the shared LDT instead
of a newly-made copy. Code factually removed LDT from the process that
did rfork(0).
Introduce user_ldt_deref() function that does decrement of refcount for
the struct proc_ldt, and call it in the rfork(0) case on the shared LDT.
Reviewed by: jhb
MFC after: 1 week