dmaengine: avoid map_cnt overflow with CONFIG_DMA_ENGINE_RAID

When CONFIG_DMA_ENGINE_RAID is enabled, unmap pool size can reach to
256. But in struct dmaengine_unmap_data, map_cnt is only u8, wrapping
to 0, if the unmap pool is maximally used. This triggers BUG() when
struct dmaengine_unmap_data is freed. Use u16 to fix the problem.

Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>