NVMe: Fix calls to dma_unmap_sg

dma_unmap_sg() must be called with the same 'nents' passed to
dma_map_sg(), not the number returned from dma_map_sg().

Signed-off-by: Nisheeth Bhat <nisheeth.bhat@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>