Each dma-buf has an associated size and it's reasonable for userspace
to want to know what it is.

Since userspace already has an fd, expose the size using the
size = lseek(fd, SEEK_END, 0); lseek(fd, SEEK_CUR, 0);
idiom.

v2: Added Daniel's sugeested documentation, with minor fixups
Signed-off-by: NChristopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NSumit Semwal <sumit.semwal@linaro.org>