loop: fix error return code in loop_add()

Fix to return a negative error code from the error handling
case, as returned elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --cc drivers/block/loop.c
Simple merge