[IA64] honor notify_die() returning NOTIFY_STOP

This requires making die() and die_if_kernel() return a value, and their
callers to honor this (and be prepared that it returns).

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>