UBUNTU: SAUCE: mm: Only expand stack if guard area is hit

UBUNTU: SAUCE: mm: Only expand stack if guard area is hit

This was a change which happened rather late in the process. It might
have some performance benefit as it avoids trying to expand the stack
every time it is touched and instead checks on whether the guard area
has been reached.

CVE-2017-1000364

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>