arm64: Use the reserved TTBR0 if context switching to the init_mm

[ Upstream commit e53f21bce4d35a93b23d8fa1a840860f6c74f59e ]

The idle_task_exit() function may call switch_mm() with next ==
&init_mm. On arm64, init_mm.pgd cannot be used for user mappings, so
this patch simply sets the reserved TTBR0.

Cc: <stable@vger.kernel.org>
Reported-by: Jon Medhurst (Tixy) <tixy@linaro.org>
Tested-by: Jon Medhurst (Tixy) <tixy@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>