map_len is unsigned. Checking map_len <= 0 is buggy when it should be
below zero. So, check exact expression instead of map_len.
Signed-off-by: NKulikov Vasiliy <segooon@gmail.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NDave Chinner <david@fromorbit.com>