x86: Suppress stack overrun message for init_task

init_task doesn't get its stack end location set to
STACK_END_MAGIC, and hence the message is confusing
rather than helpful in this case.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
LKML-Reference: <4B06AEFE02000078000211F4@vpn.id2.novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>