Move the poison pointer offset to 0xdead000000000000, a
recognized value that is not mappable by user-space exploits.

Cc: <stable@vger.kernel.org>
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NThierry Strudel <tstrudel@google.com>
Signed-off-by: NJeff Vander Stoep <jeffv@google.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>