this is an updated version of the LDT fixes. It fixes the following kinds
of problems:

 - fix a possible gcc optimization causing a race causing the loading of a
   corrupt LDT descriptor upon context switch. [this fix got simplified
   over previous versions.]

 - remove an unconditional OOM printk, and there's no need to set ->size
   in the OOM path.

 - fix preemption bugs, load_LDT()/clear_LDT() was not preemption-safe,
   when it was used outside of spinlocks.

the context-switch race is the following. 'LDT modification' is the
following operation: the seg->ldt pointer is modified, then seg->size is
modified. In theory gcc is free to reschedule the two modifications, and
first modify ->size, then ->ldt. Thus if this modification is not
synchronized with context-switches, another thread might see a temporary
state of the new ->size [which was increased], but still the old pointer.
Ie.:

	CPU0				CPU1

	pc->size = newsize;
					load_LDT(); // (oldptr, newsize)
	pc->ldt = newptr;

the corrupt LDT is loaded until the SMP cross-call is sent, leaving the
window open for many usecs.

the fix is to put a wmb() after ->ldt modifications. [this is also in
preparation of not-write-ordered SMP x86 designs.]