dma_map_sgtable internally uses the dma_map_sg_attrs helper. The problem
is that dma_map_sg_attrs returns the number of entries mapped, whereas
dma_map_sgtable returns nonzero on failure.  This leads to
dma_map_sgtable returning non-zero-but-positive values which tricks
other areas of the stack into thinking nents is a valid pointer.

This checks if nents is valid and returns zero if so, updating the nents
field in sgt. This fixes PRIME render offload with nvidia-drm.

(cherry picked from commit fa49fe56504273d8bc8fce7caf3de296debf6172)