dma-mapping: add unlikely hint to error path in dma_mapping_error

Zillions of drivers use the unlikely() hint when checking the result of
dma_mapping_error(). This is an inline function anyway, so we can move
the hint into the function and remove it from drivers over time.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>