HWPOISON: Add support for poison swap entries v2

Memory migration uses special swap entry types to trigger special actions on
page faults. Extend this mechanism to also support poisoned swap entries, to
trigger poison handling on page faults. This allows follow-on patches to
prevent processes from faulting in poisoned pages again.

v2: Fix overflow in MAX_SWAPFILES (Fengguang Wu)
v3: Better overflow fix (Hidehiro Kawai)

Signed-off-by: Andi Kleen <ak@linux.intel.com>