dma-buf: add initial vmap documentation

   Note that these calls need to always succeed. The exporter needs to complete

   any preparations that might fail in begin_cpu_access.

   For some cases the overhead of kmap can be too high, a vmap interface

   is introduced. This interface should be used very carefully, as vmalloc

   space is a limited resources on many architectures.

   Interfaces:

      void *dma_buf_vmap(struct dma_buf *dmabuf)

      void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)

   The vmap call can fail if there is no vmap support in the exporter, or if it

   runs out of vmalloc space. Fallback to kmap should be implemented.

3. Finish access

   When the importer is done accessing the range specified in begin_cpu_access,