Whenever we schedule, __switch_to calls load_esp0 which does:

	tss->esp0 = thread->esp0;

This is never initialized for the initial thread (ie "swapper"), so when we're
scheduling that, we end up setting esp0 to 0.  This is fine: the swapper never
leaves ring 0, so this field is never used.

lguest, however, gets upset that we're trying to used an unmapped page as our
kernel stack.  Rather than work around it there, let's initialize it.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>