ARM: ensure the signal page contains defined contents

commit 9c698bff upstream.

Ensure that the signal page contains our poison instruction to increase
the protection against ROP attacks and also contains well defined
contents.

Bug: 197850306
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Nobuhiro Iwamatsu (CIP) <nobuhiro1.iwamatsu@toshiba.co.jp>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 8db77dca)
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: I018db4646b4ed3a04497f8bd512a78289b0ad85c