arm64: compat: Poison the compat sigpage

arm64: compat: Poison the compat sigpage

Commit be5af58fd350 ("ARM: ensure the signal page contains defined contents")
poisoned the unused portions of the signal page for 32-bit Arm.

Implement the same poisoning for the compat signal page on arm64 rather
than using __GFP_ZERO.

Signed-off-by: Will Deacon <will@kernel.org>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20210318170738.7756-6-will@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>