Documentation DMA-API-HOWTO.txt Add dma mapping error check usage examples

Enhance the document to discuss the importance of dma
mapping error checks after dma_map_single() and
dma_map_page() calls. Also added usage examples that include
unmap examples in error paths when dma mapping error is
returned.  Includes correct and incorrect usages to high
light some common mistakes in error paths especially when
dma mapping fails when more than one dma mapping call is
made.

Signed-off-by: Shuah Khan <shuah.khan@hp.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>