[PATCH] libata: don't initialize sg in ata_exec_internal() if DMA_NONE (take #2)

Calling sg_init_one() with NULL buf causes oops on certain configurations. Don't initialize sg in ata_exec_internal() if DMA_NONE and make the function complain if @buf is NULL when dma_dir isn't DMA_NONE. While at it, fix comment. The problem is discovered and initial patch was submitted by

[PATCH] libata: don't initialize sg in ata_exec_internal() if DMA_NONE (take #2)

Calling sg_init_one() with NULL buf causes oops on certain configurations. Don't initialize sg in ata_exec_internal() if DMA_NONE and make the function complain if @buf is NULL when dma_dir isn't DMA_NONE. While at it, fix comment. The problem is discovered and initial patch was submitted by