This patch fixes the problem of mapping contigous and non contigous dma buffers.
Currently page struct is calculated from the buf->dma_addr which is not the
physical address. It is replaced by buf->pages which points to the page struct
of the first page of contigous memory chunk. This gives the correct page frame
number for mapping.
Non-contigous dma buffers are described using SG table and SG lists. Each
valid SG List is pointing to a single page or group of pages which are
physically contigous. Current implementation just maps the first page of each
SG List and leave the other pages unmapped, leading to a crash. Given solution
finds the page struct for the faulting page through parsing SG table and map it.