LoongArch: Make KASAN usable for variable cpu_vabits

Currently, KASAN on LoongArch assume the CPU VA bits is 48, which is
true for Loongson-3 series, but not for Loongson-2 series (only 40 or
lower), this patch fix that issue and make KASAN usable for variable
cpu_vabits.

Solution is very simple: Just define XRANGE_SHADOW_SHIFT which means
valid address length from VA_BITS to min(cpu_vabits, VA_BITS).

Cc: stable@vger.kernel.org
Signed-off-by: Kanglong Wang <wangkanglong@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>