dma-buf needs to support user-space mmap

While this was a non-goal for version 1 (now upstream), it has become clear from use cases on Android, V4L2, and some of the OpenGL{|ES} API that supporting user-space mmap of buffers will be an absolute requirement. This will likely require a new dma-buf op (a file op on the dma-buf file descriptor). As with other mmap requests, this would be allowed to fail, but we need to support device drivers ((in particular, exporters) in making this available.