LoongArch: Fix return value underflow in exception path

This patch fixes an underflow issue in the return value within the
exception path, specifically at .Llt8 when the remaining length is less
than 8 bytes.

Cc: stable@vger.kernel.org
Fixes: 8941e93c

 ("LoongArch: Optimize memory ops (memset/memcpy/memmove)")
Reported-by: Weihao Li <liweihao@loongson.cn>
Signed-off-by: WANG Rui <wangrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>