staging: android: ion: Add support for dma_buf_vmap

Currently ION only supports mapping memory from the kernel via
dma_buf_kmap. However dma_buf_kmap only allows mappings of page size and
clients would like an easier way to map the complete ION buffer, so add
support for dma_buf_vmap.

Change-Id: Ibff3c4b83f323676b86f558cf05c5e49d53dc4e9
Signed-off-by: Liam Mark <lmark@codeaurora.org>